chore: regenerate README (#1234)

This PR was generated using Autosynth. 🌈


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

```
2021-04-12 15:30:42,513 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigquery/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-12 15:30:43,819 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/997306e0-3d44-4e2d-ba32-9da6f2aa536e/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquery.git",
-        "sha": "21f922128419da78a8be765b45ce6bdd0f9c2472"
+        "sha": "f082628ea36eb90d0fac31336499da003b89f30a"
       }
     },
     {

README.md

@@ -19,7 +19,7 @@ See https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/The-Google
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>19.2.1</version>
+      <version>20.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -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:19.2.1')
+implementation platform('com.google.cloud:libraries-bom:20.0.0')
 
 compile 'com.google.cloud:google-cloud-bigquery'
 ```