GitHub action to automate release notes helper script

[Anish9901]

Description

Our release notes helper script: find_missing_prs.sh, is still flaky on some systems and is difficult to debug.
We should create a GitHub action that automatically

• Runs this script whenever a release branch is cut or a tag is created.
• Creates a Release notes x.x.x PR with the output of the script.
• Updates the PR if any commits are made to the release branch.

[seancolsen]

I think this is a good idea, @Anish9901.

I removed the "help wanted" label though because I don't think this would be very easy for a newcomer to work on. Specifically, I think it would be hard for someone to test and verify, and hard to understand the context of the problem without familiarity with our release process.