Fix GitHub milestone automation

[PerfectSlayer]

What Does This Do

This PR fixes the GitHub milestone automation by adding missing token permission to update both the issue and pull_request resource to add a milestone.

Motivation

Additional Notes

Related to https://github.com/DataDog/dd-trace-java/pull/7531

Contributor Checklist

• [x] Format the title according the contribution guidelines
• [x] Assign the type: and (comp: or inst:) labels in addition to any usefull labels
• [x] Squash your commits prior merging or merge using GitHub's Squash and merge
• [x] Don't use close, fix or any linking keywords when referencing an issue.
  Use solves instead, and assign the PR milestone to the issue
• [ ] Update the public documentation in case of new configuration flag or behavior

Jira ticket: VULN-8166