[8.x] mergify: support backports automation with labels (backport #16937)

[mergify]

What does this PR do?

• Enable https://docs.mergify.com/workflow for:
  • Creating backports automatically using GitHub backport labels when the original PR gets merged.
  • Notify when backports are stalled.
  • Notify when backports contain conflicts
  • Notify the backports policy (always 8.x by default unless backport-skip)
  • Create PRs even with conflicts.
• Enable copy GitHub labels from the original PR to the backported PRs. Mergify does not support this feature out of the box, see github-actions: support for copying the backport labels apm-server#15190
• Enable merge conflict detection. See github-actions(pre-commit): detect merge conflicts and shebangs apm-server#15319
• Enable Mergify rules when https://github.com/elastic/logstash/actions/workflows/version_bumps.yml runs with a minor.
• Enable GitHub backport label creation when https://github.com/elastic/logstash/actions/workflows/version_bumps.yml runs with a minor.

Why is it important/What is the impact to the user?

Use the same backport mechanism as done in some other projects. While still support for the former approach.

Tasks

• Wait for Mergify to be installed
• Create GitHub labels

Follow-ups

• Delete https://github.com/elastic/logstash/blob/main/.github/workflows/pr_backporter.yml
• Delete https://github.com/elastic/logstash/blob/main/devtools/backport

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Relates #16933

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #16937 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d40386a has failed:

On branch mergify/bp/8.x/pr-16937
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit d40386a3.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .ci/Makefile
	new file:   .github/workflows/mergify-labels-copier.yml
	new file:   .github/workflows/pre-commit.yml
	modified:   .github/workflows/version_bumps.yml
	new file:   .mergify.yml
	new file:   .pre-commit-config.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .buildkite/pull-requests.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 4739e4f

cc @v1v

[mergify]

This pull request has not been merged yet. Could you please review and merge it @v1v? 🙏

[mergify]

This pull request has not been merged yet. Could you please review and merge it @v1v? 🙏