chore: regenerate README (#778)

This PR was generated using Autosynth. 🌈


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

```
2021-04-22 15:43:00,856 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-22 15:43:02,769 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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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-bigtable.git",
-        "sha": "5ab60eedb7104d8d2b3bddc72db0a27576a65395"
+        "sha": "ac534a6ef7c08c8bf6ecb94c590643bd473baa9d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "b33b0e2056a85fc2264b294f2cf47dcd45e95186"
+        "sha": "5a6f30b8bb7672cf237c4f75d5aa96bfba170e7f"
       }
     }
   ]

README.md

@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-bigtable'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-bigtable:1.23.0'
+compile 'com.google.cloud:google-cloud-bigtable:1.23.1'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.23.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.23.1"
 ```
 
 ## Authentication