Fix auto milestone action (#18731)
Summary of the issue:
The auto milestone action has been broken since #18396.
Description of user facing changes:
None
Description of developer facing changes:
Pull requests should have a milestone automatically applied on merge.
Description of development approach:
Gave the action write permission for issues.
This is needed because PRs are considered to be a type of issue by GitHub. The action still needs write permission for pull requests as only users with merge permissions can add a milestone to pull requests.
