Automate post release process (#4981)

Motivation:

Automate the remaining release process to relieve the effort of the
devs.

Modifications:

- Add `post-release.ts` that does some tasks after releasing a new
version:
  - Close milestone.
  - Write release notes.
  - Create a pull request to Central Dogma for updating Armeria version.
- Make `post-release.ts` triggered after finishing the `publish` task.

Result:

Enhanced release workflows.