ci: release-please.yml to tag pull request number (#5759)

This repository creates multiple Git tags upon one release pull request
(commit). Example:
#5696

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

Author: suztomo

.github/release-please.yml

@@ -1,3 +1,3 @@
 handleGHRelease: true
 manifest: true
-
+tagPullRequestNumber: true