[Tooling] Release-on-CI automation improvements

[iangmaia]

This PR implements a few additions for our release automation on CI:

• Automatically move PRs from the current release milestone to the next one
• Uses the shared create_release_backmerge_pull_request Action from release-toolkit, assigning a milestone to the backmerge PR.
• Make sure release tasks won't fail due to milestones already being closed
• Annotate builds triggering other builds with their URL
• Close hotfix milestone
• Updated release pipelines to not allow manual retry
• Added ensure_branch_does_not_exist in the beginning of code_freeze, to avoid unexpected behavior and errors.
• ~Removed the :trigger_ testing lanes, as testing can happen either via Rv2 or the Buildkite web UI.~ (moved to the PR https://github.com/wordpress-mobile/WordPress-iOS/pull/23589)

Update:

• Merged https://github.com/wordpress-mobile/WordPress-iOS/pull/23592 into this PR (without realizing it wasn't for trunk), so it now also contains release publishing.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23585-536b1c0
	Version	25.3
	Bundle ID	com.jetpack.alpha
	Commit	536b1c0bb2edd7b1524f4527f5ec764c07742cc5
	App Center Build	jetpack-installable-builds #9758

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23585-536b1c0
	Version	25.3
	Bundle ID	org.wordpress.alpha
	Commit	536b1c0bb2edd7b1524f4527f5ec764c07742cc5
	App Center Build	WPiOS - One-Offs #10715

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.