Replace automerge bot with github's built-in automerge

[Strilanc]

Github now has native automerge functionality: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request

We should turn this on for the cirq repo, check that it works the way we want, and then turn off cirq bot.

[MichaelBroughton]

Just tried it. Automerge works, but it won't see auto update the branch for you, so it's still up to the PR creator to do that. Has anyone found any way to do this ? Otherwise I'd be inclined to stick with the Cirq Bot.