build: enable onDemand for release-please (#12708)

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