ci: Create backport pr based on milestone.

[Yaminyam]

This repository manages a few release branches where latest bug fixes and minor improvements are backported and released as a series of prior versions.
(e.g., The main branch targets 24.09, while fixes are backported to 24.03 and 23.09 and shipped as their patch releases.)

So far, we have created the backport commits by manually git cherry-pick, adding several git trailers like Backported-from: main (24.09), Backported-to: 24.03.

In this PR, we are going to let GitHub Actions auto-generate the backport PRs. The target branches are calculated from the original PR's milestone property. Compared to individual cherry-picked commits, it has the following advantages:

• As being a PR, we can make all required CI checks to be properly applied before pushing to the target release branch.
• The backport PR could explicitly show what's broken in terms of both the code-level merge conflicts.
• We could automate the merge if there is no code conflicts and all CI succeeds ("Enable auto-merge").
• We could have an extra chance to review and update the backported change if it cannot be auto-merged.
• When there are multiple target branches to backport, we can reduce the human labor to create backport PRs/commits to all of them.

Sometimes git cherry-pick may fail due to merge conflicts. In such case, the GitHub Actions workflow will append a comment to the original PR indicating that a human developer should take care of manual cherry-picking.

The workflow also automatically combines the git trailers including those already existing in the original PR's merge commit, those added by GitHub (e.g., Co-authored-by), and our own backport trackers (e.g., Backported-from, Backported-to, and a new Backport-of field).

The result backport PR is composed like the followings:

• The title: {original-merge-commit-header} (#{backport-PR-number})
  For example, if the original PR's merge commit has the headline: "fix: A bug (#123)", the backport PRs will have the titles like "fix: A bug (#123) (#124)".
• The body: "This is an auto-generated backport PR of #{original-PR-number} to the {target-milestone} release."
• It has a single cherry-picked commit.

[achimnol]

Sion has tested most functionalities in his test repo.
Let's merge this and see if it goes well.

[achimnol]

Sion has tested most functionalities in his test repo.
Let's merge this and see if it goes well.

[github-actions]

Backport to 24.09 is failed. Please backport manually.