chore(java): update release_type choices to stable and preview (#1331)

* chore(java): update release_type choices to stable and preview

* chore: update badge names
Source-Link: googleapis/synthtool@d10357a
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:9669c169d0582f13d6b2d319a43a78fc49f296a883aa48519bd0e5c7d34087c4

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:7f737996c88586c499f7b0c25294c760f9f5eec092289a0e08d2d3f42a069ebb
+  digest: sha256:9669c169d0582f13d6b2d319a43a78fc49f296a883aa48519bd0e5c7d34087c4

README.md

@@ -340,7 +340,7 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [kokoro-badge-link-4]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-bigquery/java8-win.html
 [kokoro-badge-image-5]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-bigquery/java11.svg
 [kokoro-badge-link-5]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-bigquery/java11.html
-[stability-image]: https://img.shields.io/badge/stability-ga-green
+[stability-image]: https://img.shields.io/badge/stability-stable-green
 [maven-version-image]: https://img.shields.io/maven-central/v/com.google.cloud/google-cloud-bigquery.svg
 [maven-version-link]: https://search.maven.org/search?q=g:com.google.cloud%20AND%20a:google-cloud-bigquery&core=gav
 [authentication]: https://github.com/googleapis/google-cloud-java#authentication