Add merge-trunk-develop-pr action

[tomalec]

Changes proposed in this Pull Request:

Add merge-trunk-develop-pr action to create a PR to merge trunk to develop after a release done by woocommerce/grow/prepare-extension-release. As automerging with automerge-released-trunk may not work with branch protections.

Screenshots:

Detailed test instructions:

1. Create a test repo that has the woocommerce/grow/prepare-extension-release set up.
2. Add to trunk another workflow following the README https://github.com/woocommerce/grow/blob/add/merge-trunk-develop-pr/packages/github-actions/actions/merge-trunk-develop-pr/README.md but with the prereleased version actions-v1.9.1-pre

name: PR to merge a released trunk

on:
  pull_request:
    types:
      - closed
    branches:
      - trunk

jobs:
  automerge_trunk:
    runs-on: ubuntu-latest
    steps:
      - uses: woocommerce/grow/merge-trunk-develop-pr@@actions-v1.9.1-pre

3. Create a develop branch
4. Create a release with woocommerce/grow/prepare-extension-release. Then merge it - check that a PR to merge trunk to develop gets created. Like
   • https://github.com/tomalec/grow/pull/27
   • https://github.com/tomalec/grow/actions/runs/5742320916?pr=27
   • https://github.com/tomalec/grow/pull/43
5. Create a release with woocommerce/grow/prepare-extension-release, switch a target branch to something else. Merge the PR. No PR should get created
6. Create a release with woocommerce/grow/prepare-extension-release, close it without merging. No PR should get created
7. Create a PR with release/1.2.3 as a regular user, then merge it to trunk. No PR should get created

Additional details:

• Probably after merging this PR, we could tweak the PR checklist to mention using merging the PR created by this action, if applicable

Changelog entry

Add merge-trunk-develop-pr action

[eason9487]

Thanks for the work. LGTM.

This new GH action could be added to this list as well. (Maybe also add automerge-released-trunk action by the way?)

All my comments are not blocking this PR.

[eason9487]

I'm not pretty sure but maybe these two steps are not necessary because the next step doesn't require any repo files nor operates with git commit. Perhaps it's worth trying to see if this action still works without them or at least the second one.

[tomalec]

Thanks :) I thought GHA needed an explicit username to create PRs.
But Github API calls works out of the box, away from CLI git https://github.com/tomalec/grow/actions/runs/5787573264?pr=30 :)

Removed in 161b44a

[tomalec]

Thank you for the review, and great suggestions. Applied them all.