chore: regenerate README (#1286)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-07-06 17:54:31,102 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-06 17:54:32,542 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/95ceab1a-c04c-4851-9560-24473e8d1bc6/targets

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "2f26474b24ccf556523c3bceabd88fab43b15085"
+        "sha": "9340b8bb59014656446a35ace9b0d786c67582d7"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "333fd90856f1454380514bc59fc0936cdaf1c202"
+        "sha": "39652e3948f455fd0b77535a0145eeec561a3706"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.6.0</version>
+      <version>20.7.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -38,25 +38,25 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>6.7.0</version>
+  <version>6.8.0</version>
 </dependency>
 
 ```
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.6.0')
+implementation platform('com.google.cloud:libraries-bom:20.7.0')
 
 compile 'com.google.cloud:google-cloud-spanner'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:6.7.0'
+compile 'com.google.cloud:google-cloud-spanner:6.9.1'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.7.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.9.1"
 ```
 
 ## Authentication
@@ -239,6 +239,7 @@ has instructions for running the samples.
 | Create Database With Encryption Key | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithEncryptionKey.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithEncryptionKey.java) |
 | Create Database With Version Retention Period Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithVersionRetentionPeriodSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithVersionRetentionPeriodSample.java) |
 | Create Instance Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateInstanceExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateInstanceExample.java) |
+| Create Instance With Processing Units Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateInstanceWithProcessingUnitsExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateInstanceWithProcessingUnitsExample.java) |
 | Custom Timeout And Retry Settings Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CustomTimeoutAndRetrySettingsExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CustomTimeoutAndRetrySettingsExample.java) |
 | Get Commit Stats Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/GetCommitStatsSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/GetCommitStatsSample.java) |
 | Query With Numeric Parameter Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/QueryWithNumericParameterSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/QueryWithNumericParameterSample.java) |