Add automation to automatically assigned milestones to issues closed during iteration

[karrtikr]

Ideally we only want to add milestones for issues which have an associated PR.

Together with #21934, this should make sure that an issue fixed is always verified.

[karrtikr]

@lramos15 We found something similar in https://github.com/microsoft/vscode-github-triage-actions/blob/stable/release-pipeline/index.ts but it also assumes the concept of "unreleased" labels, which we don't have.

Also it seems like https://github.com/microsoft/vscode-github-triage-actions/blob/71a4da6d2999defd04415ec9924c0579ad26ac90/release-pipeline/ReleasePipeline.ts#L81 calls into "issue.getClosingInfo()" which assumes we're working on vscode repository:

https://github.com/microsoft/vscode-github-triage-actions/blob/71a4da6d2999defd04415ec9924c0579ad26ac90/api/octokit.ts#L446-L461

Can you help with how to modify it to work with this repository? What we want to do is add the current milestone only if an issue is closed via a PR.