chore: regenerate README (#1436)

This PR was generated using Autosynth. 🌈


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

```
2021-07-08 18:34:28,544 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigquery/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-08 18:34:29,847 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/81d2ea6c-8e20-4fbf-9fdc-40a1d9147e9d/targets

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

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquery.git",
-        "sha": "e4dcd1a0a8b7e38ef8e366f4fe0e3ef306c3889f"
+        "sha": "9d67e05d2a6fab0c9e6017dec33b0d74ad821825"
       }
     },
     {

README.md

@@ -47,7 +47,7 @@ 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:20.7.0')
+implementation platform('com.google.cloud:libraries-bom:20.8.0')
 
 compile 'com.google.cloud:google-cloud-bigquery'
 ```