Notify after 2 weeks and close PRs as stale after 4 weeks of no activity

## Summary
Send a message after 2 weeks to say we'll close. Then close PRs as stale after 4 weeks of no activity

## 💬 Description
Use an Action such as: https://github.com/actions/stale to mark PRs that have had no activity (updates or comments) in the last 2 weeks, leaving them with a message to why, thanking them for their contribution and suggest to them to re-open it if they have further improvements. This is not required for issues, PRs only.

## 💰 Use value
Keep on top of long-lasting stale PRs.

Start by reviewing the repository's GitHub Actions workflows and the actions/stale documentation linked in the issue. Configure pull requests to receive a warning after two weeks of inactivity and close after four weeks, with a message explaining the policy and how to reopen; verify that issues are excluded.

Written by the indexing model from the issue text.