chore: cleanup release-please config (googleapis#2315)

- Remove redundant options already declared at the top level.\n- Remove bumpMinorPreMajor for repositories after the first major release.

Author: diegomarquezp

.github/release-please.yml

@@ -2,13 +2,7 @@ releaseType: java-yoshi
 bumpMinorPreMajor: true
 handleGHRelease: true
 branches:
-- branch: 1.21.x
-  releaseType: java-yoshi
-  bumpMinorPreMajor: true
-  handleGHRelease: true
-- branch: protobuf-4.x-rc
-  releaseType: java-yoshi
-  bumpMinorPreMajor: true
-  handleGHRelease: true
-  manifest: true
+    - branch: 1.21.x
+    - branch: protobuf-4.x-rc
+      manifest: true
 extraFiles: ["README.md"]