chore: regenerate README (#886)

This PR was generated using Autosynth. 🌈


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

```
2021-06-23 15:24:27,438 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-23 15:24:28,701 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/631627ed-b485-48f2-b12a-6df2298117f5/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": "644341b5ebe3061d2b4fe341ec30ca761b7de9d3"
+        "sha": "7e2c3427b29cd22a6caccf7f4409ac5cd2f6c4ee"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "2514a9985b36626acb384dff903c62ac5963441b"
+        "sha": "9763f20e4b7bb1091082462b2f7970e965d0d414"
       }
     }
   ]

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>20.6.0</version>
+      <version>20.7.0</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:20.6.0')
+implementation platform('com.google.cloud:libraries-bom:20.7.0')
 
 compile 'com.google.cloud:google-cloud-bigtable'
 ```