build: enable onDemand for release-please

The monorepo has a lot of commits and overloads release-please by triggering it on every commit. Enabling onDemand will skip automatic processing on push events and require manual triggering via the release-please:force-run label.

Author: meltsufin

.github/release-please.yml

@@ -2,6 +2,7 @@ primaryBranch: main
 releaseType: java-yoshi
 manifest: true
 handleGHRelease: true
+onDemand: true
 branches:
     - branch: 1.0.x
       releaseType: java-backport