Improve the GitHub actions workflow for releasing

[vaurdan]

Describe the bug

After the 3.17 release - the first time using our new workflow - we've identified a few minor issues that should be addressed before the next release:

• Generated PR has an empty space before the first line of the changelog ( ### Added)
• Generated PR hasn't a set milesone
• GitHub action bot not signing commits / PRs (we might need our own bot user for that)
• When running with dry-run, if the tag already exists it is deleted locally, but when pushing the tag it fails since it already exists on the remote.
• GitHub release is not published automatically when the zip file is uploaded

Nice to have / Improvements

• When running the release workflow without dry-run set, it might optionally attempt to remove any old release and tag created by the dry-run execution.
• When the Update version number and changelog for x.x.x release PR is merged, maybe trigger automatically an action that creates a Release wp-parsely x.x.x PR that merges develop into trunk.
• After the release, automatically create a Merge trunk into develop after the wp-parsely x.x.x release PR.

[acicovic]

After releasing v3.18, here are my comments:

• We still have a couple of spaces before ## Added (can be seen here when editing the PR description).
• I can confirm that the GitHub release is being published as a draft even for non-dry runs, and that you need to go to the release and publish it manually.