Allow only "squash and merge" from GitHub UI

Github has three possible merging modes from the UI:

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/about-pull-request-merges

Of those, we want "squash and merge" 99% of the time, but sometimes other methods are chosen, resulting in a less than ideal commit graph.

This is configurable via .asf.yml : 

https://cwiki.apache.org/confluence/display/INFRA/Git+-+.asf.yaml+features#Git.asf.yamlfeatures-Mergebuttons

I propose that we disable the "merge" and "rebase" options for every Phoenix repo via the .asf.yml file.

This only applies to the GitHub Web UI, the other methods can still be used via the standard git operations.