Replace bulldozer with github auto-merge

# 🎯 Problem to be solved

Bulldozer bot for auto merging did seem irresponsive sometimes and is yet another external tool that we don't need. We can move to [github auto-merge](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request).
