chore: regenerate README (#1308)

This PR was generated using Autosynth. 🌈


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

```
2021-07-21 02:23:32,513 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-21 02:23:33,819 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/dc8ddd2d-54cb-4d87-a0fa-343e70b3b4ff/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.git",
-        "sha": "d72c2f79f8cf0b83da00060587a079ce859c87a2"
+        "sha": "fa0c65dc31236e05e6b10508281cf58e82ee87ef"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4124d245133c0f740ac5e6fa2e7daeba327f1297"
+        "sha": "1563597d28eca099d6411bbc29ecd09314a80746"
       }
     }
   ]

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>6.9.1</version>
+  <version>6.10.0</version>
 </dependency>
 
 ```
@@ -51,12 +51,12 @@ 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:6.9.1'
+compile 'com.google.cloud:google-cloud-spanner:6.10.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.9.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.10.0"
 ```
 
 ## Authentication