Update release automation

[trask]

#11528 introduced a new version of japicmp which includes the version numbers in the diffs

to make the automation work with this new version, need to

• prepare-release-branch.yml - need to incorporate ./gradlew jApiCmp into both PRs that it creates since version being updated in each one
• release.yml - need to push a new PR to update the diffs after the release hits maven central (since that's when the base version in the diffs will get updated)