chore: regenerate README (#1530)

This PR was generated using Autosynth. 🌈


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

```
2021-08-24 23:30:16,489 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigquery/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-24 23:30:17,732 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/a2bd1d2d-d8a7-46fd-ad1f-a2ff282ddbaf/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquery.git",
-        "sha": "e1713f2cf04349cd0336cd3aca7ca2870bfa96d4"
+        "sha": "da09e53bbdd18693c182d7945506b0531bb445e4"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8032ab5a32471d85ece8fc261df16cc40fbfa4fb"
+        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
       }
     }
   ]

README.md

@@ -21,7 +21,7 @@ See https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/The-Google
     <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>