Allow auto-merge on PRs (open-telemetry#6812)

This seems nice for after pushing `spotlessApply` on an otherwise
approved and passing PR. I just enabled it and tried it on open-telemetry#6774.

(somewhat related to open-telemetry#6743)

Btw, I thought this was helpful explanation

> After you enable auto-merge for a pull request, if someone who does
not have write permissions to the repository pushes new changes to the
head branch or switches the base branch of the pull request, auto-merge
will be disabled. For example, if a maintainer enables auto-merge for a
pull request from a fork, auto-merge will be disabled after a
contributor pushes new changes to the pull request.


https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request#about-auto-merge

docs/contributing/repository-settings.md

@@ -7,6 +7,8 @@ to https://github.com/open-telemetry/community/blob/main/docs/how-to-configure-n
 
 * Allow squash merging > Default to pull request title and description
 
+* Allow auto-merge
+
 * Automatically delete head branches: CHECKED
 
   So that bot PR branches will be deleted.