GitHub Actions: implement automation for unassigning abandoned issues

[kyleecodes]

Overview

Implement a GitHub Action that automatically unassigns contributors from an issue after days of inactivity.
Starting out, we will do 3 months of inactivity, but will adjust as needed.

Action Items

• Draft a helpful message tagging the assignee.
• Design plan for the GitHub Action, document in internal docs.
• Write a new workflow and submit a PR.

Resources

https://docs.github.com/en/actions/using-workflows/creating-starter-workflows-for-your-organization
https://docs.github.com/en/actions/managing-issues-and-pull-requests/using-github-actions-for-project-management
https://github.com/marketplace/actions/unassign-contributor-after-days-of-inactivity
https://github.com/actions/stale
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows

[kyleecodes]

Done: 3704012