[Tooling] Move Danger to GitHub Actions

[iangmaia]

This PR experiments moving the current Buildkite step setup to GitHub Actions (discussion on paaHJt-5Qn-p2).

Running on Buildkite is still an option, but it would require a GitHub action (or an intermediate server) to forward to Buildkite Pull Request Events such as "labeled", "unlabeled", "milestoned" and so on. This was implemented as a prototype on woocommerce/woocommerce-android#10385.

If we find problems using GitHub Actions, we can always revert this change and go back to Buildkite, as the setup works in general (just not as fast and as simple as GHA alone).

How to test

Make sure CI is green and Danger runs on labels/milestones/reviewers changes as well as on code changes.

[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	pr22276-5aa18cf
	Version	24.0
	Bundle ID	org.wordpress.alpha
	Commit	5aa18cf
	App Center Build	WPiOS - One-Offs #8424

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 Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22276-5aa18cf
	Version	24.0
	Bundle ID	com.jetpack.alpha
	Commit	5aa18cf
	App Center Build	jetpack-installable-builds #7447

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

[mokagio]

Hey @iangmaia 👋 You might notice a merge trunk commit here from me. That is to bring this branch up to date with the latest CI changes on trunk and ensure everything works on the new Xcode 15.1 CI stack.

Don't hesitate to ping me if there's any trouble.

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

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