Events that trigger workflows for pull_request_target list is missing milestoned+demilestoned

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request_target

What part(s) of the article would you like to see updated?

Add milestoned and demilestoned to match https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request

Additional information

• Add missing activity types for pull_request and pull_request_target actions/languageservices#242 fixed the corresponding code that's used by Visual Studio Code