[Tooling] Use `buildkite_pipeline_upload` action to inline Buildkite release pipelines

[iangmaia]

Description

iOS counterpart: https://github.com/woocommerce/woocommerce-android/pull/12839 Related: pdccJN-xR-p2

This PR uses the release-toolkit action buildkite_pipeline_upload implemented on https://github.com/wordpress-mobile/release-toolkit/pull/597.

Our release pipelines (such as complete-code-freeze, finalize-release, etc) usually require to start a separate pipeline to make a release build. The changes in this PR will result in add the steps of a release pipeline to the currently running build, without starting a new build altogether.

I've also added a trigger to run Danger whenever a PR moves to the "Ready For Review" state, as previously we relied on this behavior from Buildkite, but now it has changed.

Testing

I've added (and later reverted) a test commit ebf9a03b2f8b3c0417533531369f2efa0860c9de to test the complete_code_freeze lane.

These are resulting test builds created using the Buildkite Web UI directly with PIPELINE=release-pipelines/complete-code-freeze.yml:

• https://buildkite.com/automattic/woocommerce-android/builds/24827
• https://buildkite.com/automattic/woocommerce-android/builds/24828 (with INCLUDE_WEAR_APP=true set on Buildkite)

This PR can be tested in a similar way to the test above: creating a branch off this one, slightly modifying a release lane to prevent side effects and running the build directly on Buildkite.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	e5aa4651beadaff84b6b28435d0119d5291ff52e
	Direct Download	woocommerce-wear-prototype-build-pr12839-e5aa465.apk

[wpmobilebot]

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

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	e5aa4651beadaff84b6b28435d0119d5291ff52e
	Direct Download	woocommerce-prototype-build-pr12839-e5aa465.apk

[wpmobilebot]

Version 21.0 has now entered code-freeze, so the milestone of this PR has been updated to 21.1.

[iangmaia]

@AliSoftware @mokagio FYI, I've added the commit 7fef212f78f884d89e0430d8bf4b71b53a28ebdf to handle the situation highlighted by @mokagio here, so now the release pipeline should also checkout the release branch. While at it, I've changed the checkout script to receive the RELEASE_VERSION an argument as we do on iOS, for consistency.