chore: regenerate README (#683)

This PR was generated using Autosynth. 🌈


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

```
2021-03-19 18:08:33,756 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-19 18:08:34,705 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/aac9074d-9f09-4f10-950c-7d9d91914965/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigtable.git",
-        "sha": "db2229e65ab6724cdce64bbd4db6a415b0ea38ab"
+        "sha": "dc3d2071ec07fb153c8d502551e681dbb2a1767c"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "78437c732a60c64895778697b078497b0988346c"
+        "sha": "8b13da943497f75c5303700bff5aabdd85d939e0"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>19.1.0</version>
+      <version>19.2.1</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -45,7 +45,7 @@ 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:19.1.0')
+implementation platform('com.google.cloud:libraries-bom:19.2.1')
 
 compile 'com.google.cloud:google-cloud-bigtable'
 ```