chore: regenerate README (googleapis#992)

This PR was generated using Autosynth. 🌈


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

```
2021-09-07 21:10:42,032 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-09-07 21:10:43,098 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/338b2de6-4bcd-4e73-87bc-7de6c695de2d/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-bigtable.git",
-        "sha": "9b2279881c465f5e00493927e012e2195458309e"
+        "sha": "148a928e3f8f6d1e2ee1d1b119ca820d676802fd"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ad7fd76e17bf9494a3e47ff9cf445f61564432e0"
+        "sha": "17ee6e5c08f2eb47029beee4776ce990e56b3925"
       }
     }
   ]

README.md

@@ -49,7 +49,7 @@ If you are using Maven without BOM, add this to your dependencies:
 If you are using Gradle 5.x or later, add this to your dependencies
 
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:22.0.0')
+implementation platform('com.google.cloud:libraries-bom:23.0.0')
 
 implementation 'com.google.cloud:google-cloud-bigtable'
 ```