ci/treefmt: add yamlfmt

Most workflow files are already well formatted, but to make it easier to
keep it that way, we can add yamlfmt.

I personally have a preference for non-indented arrays for YAML, but
wanted to avoid bigger diffs here - the status-quo clearly are indented
arrays.

Some changes are made manually to the get-merge-commit action and the
issue templates. Those would otherwise make yamlfmt misbehave on those.

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