Deprecate probot in favor of gitbot

[gtsiolis]

Problem to solve

We've been using probot for marking stale or closing issues and PRs. This means we have to maintain two different tools for some actions that could be possible with a single tool. For example, see gitpod-io/gitpod#5898.

Proposal

Given the possibilities of gitbot we could deprecate probot and move all existing logic from .github/stale.yml to this repository.

Benefits

1. Have a single source of truth for groundwork automation, issue triaging, and more.
2. Encourage more documentation, feature requests, and bug reports in a single repository.
3. ...