chore: regenerate README (#738)

This PR was generated using Autosynth. 🌈


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

```
2021-08-19 17:51:08,918 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-19 17:51:10,105 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/2f9be49b-6ea3-43e1-b39b-900cf624eda6/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "d71de050d71f84be1fb7fdd33100c4b85d8260d7"
+        "sha": "71ecfece4144f2c1b9065c45643304509814aa35"
       }
     },
     {

README.md

@@ -19,7 +19,7 @@ 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>20.9.0</version>
+      <version>21.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>