ci: release-please.yml to tag pull request number

[suztomo]

This google-api-nodejs-client repository creates multiple Git tags upon one release pull request (commit). Example: #3650 (comment)

To adopt the new release pipeline based on Git tags, this tagPullRequestNumber creates an additional release-please-<pr number> tag to a release. We'll monitor that tag to trigger the release job. This doesn't change existing release tags.

Background: https://docs.google.com/document/d/1Y0Fv-sxLqYmjnXHry3WDq4XNPeQP1AEmsB83hNRCEY0/edit?usp=sharing&resourcekey=0-I2m8Z-mTD0-HY0TpkblOTQ

This design has been working good for google-cloud-go: https://github.com/googleapis/google-cloud-go/blob/main/.github/release-please.yml#L4

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[suztomo]

@sofisl I remember we stopped "OwlBot Post Processor" for pull requests based on a forked-repository. Would you temporarily remove "OwlBot Post Processor" from the required checks?