chore: regenerate README (#969)

This PR was generated using Autosynth. 🌈


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

```
2021-08-25 17:31:44,567 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-25 17:31:45,755 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/4356bb30-a578-486e-85fc-0be58e52bae0/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigtable.git",
-        "sha": "8b1ea8f874b3492ae976a6e17a082db84139ccbf"
+        "sha": "782ffabcdefe4990449d17b03a431ea7f04ce24d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a01e1a0bf70754f51450958b966bd673945d1e6e"
+        "sha": "e2aa5bcc3356d9e3b8b53de3b5c86226447d3a22"
       }
     }
   ]

README.md

@@ -41,7 +41,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-bigtable</artifactId>
-  <version>2.0.1</version>
+  <version>2.1.0</version>
 </dependency>
 
 ```
@@ -56,13 +56,13 @@ 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:2.0.1'
+compile 'com.google.cloud:google-cloud-bigtable:2.1.0'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "2.0.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "2.1.0"
 ```
 
 ## Authentication