chore: regenerate README (#391)

This PR was generated using Autosynth. 🌈


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

```
2021-03-11 00:12:52,132 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-11 00:12:53,033 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/f41b860a-107b-4d47-94bd-9b15ac08bdd9/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "a7c44b0837caaa42933f252249eac938eeb0ef11"
+        "sha": "b83537e08c9f453904ddbe0d1b642477a45c16ac"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8c5628b86cfa8386de7b8fc1675e6b528b552d57"
+        "sha": "ac8f20f12e7a4c0b0ae1c6fa415f684a25ea82b7"
       }
     }
   ]

README.md

@@ -17,18 +17,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner-jdbc</artifactId>
-  <version>1.20.1</version>
+  <version>1.21.0</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:1.20.1'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:1.21.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.20.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.21.0"
 ```
 
 ## Authentication