test: set up test release candidate branches (#1040)

* test: set up test releaase candidate branches

This repo has a small blast radius for release testing.
We aim to test the rc`n` versioning schema soon to be provided by googleapis/release-please#2622.

Additionally, we test with `prerelease` to understand its behavior.

* chore: use prerelease mode only

Removed prerelease settings and adjusted branch for release.

* chore: use base flags

Author: diegomarquezp

.github/release-please.yml

@@ -6,3 +6,7 @@ branches:
   handleGHRelease: true
   releaseType: java-yoshi
   branch: java7
+- bumpMinorPreMajor: true
+  handleGHRelease: true
+  releaseType: java-yoshi
+  branch: test-protobuf-4.x-rc