chore: regenerate README (#551)

This PR was generated using Autosynth. 🌈


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

```
2021-08-12 20:14:23,342 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-12 20:14:24,558 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/16f91a02-9987-4387-964e-2ee273dbe88d/targets

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Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "68125c4d896a24f186d90648d3be64b5ad15aa37"
+        "sha": "2639e40ebbde19966653f06f8b664106568f6bac"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "3d32990b6f4b57be77455e3ca89e6e21cd0c06b3"
+        "sha": "ff01716e16d2c6e87eaf87197b753ac9fcbbed5d"
       }
     }
   ]

README.md

@@ -19,20 +19,20 @@ 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>2.2.6</version>
+  <version>2.3.1</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:2.2.6'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:2.3.1'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.2.6"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.3.1"
 ```
 
 ## Authentication