Fix add-milestone GHA job

[abhishekrb19]

A GitHub action to auto-add milestones were added in #18935

As @kgyrtkirk noted in this comment, this check is failing with a 403 error for all PRs coming from forks once they are merged into master:

{"message":"Resource not accessible by integration","documentation_url":"https://docs.github.com/rest/issues/issues#update-an-issue","status":"403"}

Given the 403, it does indicate a permissions issue likely with the GitHub token used for this. Trying pull_request_target instead of pull_request similar to the existing labeler action which uses the same. Since this specific add-milestone check doesn't export any credentials, I believe using pull_request_target should be okay. Hopefully, with the elevated base token, this should work. 🤞

[capistrant]

🤞

[abhishekrb19]

Thanks for the review, @capistrant!

The milestone has been successfully added now. We'll still need to manually add the 37.0.0 milestone for existing PRs where it wasn’t added bc of the 403, but this should work going forward for new PRs.