Project Management: Milestone It: Read version from master branch package.json

[aduth]

This pull request seeks to improve the Milestone It custom GitHub action to ensure that the version read from package.json is that of the master branch, not the "current" branch of the pull request being merged. Since the branch of a pull request which has been merged may be stale, the version may not reflect the next release. The version should always be derived based on the master branch at the time of the merge.

In the course of implementing this, a few other necessary changes were made:

• Milestones should only be applied when a pull request is merged to the master branch. This restriction did not exist previously, so it was possible branches merged to other non-master branches would have a milestone applied.
• Changed "Skip" exits to use a neutral status code (documentation)

[youknowriad]

Looks good. Best way to try is to ship :)

[aduth]

Oh, I also meant to link to a few resources which led me to this change:

An example pull request which assigned to the wrong milestone: #16242 (assigned 6.0, should have been 6.1)

In the source branch package.json, the 5.9.x version can be seen, hence why it's assumed 6.0 was selected: https://github.com/faazshift/gutenberg/blob/fix/shortcode-replace/package.json

The GitHub Actions "Accessing the runtime environment" documentation appears to confirm that the source will reflect the branch of the pull request being merged:

/github/workspace: The working directory of the Docker container. GitHub Actions execute in this directory. The path to this directory is set in the GITHUB_WORKSPACE environment variable. This directory contains a copy of the repository (with version GITHUB_SHA) that triggered the workflow. [...]

GITHUB_SHA: The commit SHA that triggered the workflow.