chore: regenerate README (googleapis#412)

This PR was generated using Autosynth. 🌈


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

```
2021-03-25 01:52:12,889 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-25 01:52:14,048 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/308f86e5-3e08-4174-95a6-ede4c9611bfd/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "3e0fbd1706fc269cd7bfcd2258181487cc40cece"
+        "sha": "f0cd180f7a34c28460ff88699614cf1173205002"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "86ed43d4f56e6404d068e62e497029018879c771"
+        "sha": "bb854b6c048619e3be4e8b8ce8ed10aa74ea78ef"
       }
     }
   ]

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>1.21.0</version>
+  <version>2.0.0</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:1.21.0'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:2.0.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.21.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.0.0"
 ```
 
 ## Authentication