[BEAM-9833] Fix .asf.yaml issues, sort labels and disable rebase button

[iemejia]

Issues reported by yamllint and some minor fixes. Also set merge button as the only strategy because we don't want to encourage (o even make possible the other two).

R: @pabloem
CC: @ibzib

[robertwb]

I thought consensus was that we did want to encourage squash for those PRs with a huge pile of fixup commits (and otherwise no semantically meaningful commits).

[iemejia]

@robertwb Agree, a consensus that nobody respects :). In the case of this PR I set into the merge approach because we have only three options and the other two (squash and rebase) do not create the additional merge commit required by the Beam committer guide.

[pabloem]

I think squash is used by many, and it facilitates receiving contributions without the extra round trip to the contributors. I think we need to discuss more before removing it. Wdyt @iemejia?

[iemejia]

I am ok with squashing if it creates extra commits, but it does not seem to be the case or does it create the additional extra merge commit? I just want to ensure we follow the rules.

Now if the goal is to change the rules maybe we should move the discussion to the ML.
Pinging @kennknowles who created our default policy (extra Merge commit) to see what he thinks.

[robertwb]

OK, the rest of this change looks good. Let's get this in, disabling rebase but keeping the others, and taking squash+merge to the list as a separate discussion (and PR if need be).

[iemejia]

Done the changes suggested by @robertwb let only rebase disabled. Can someone PTAL. Thanks

[pabloem]

LGTM!