Fix merge defaults for Dataset._setitem_ and Dataset.update

[dhruvak001]

• Closes Better "merge" defaults for Dataset.__setitem__ and Dataset.update #10585
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Updates dataset_update_method to use join="left" and compat="override" instead of the expensive defaults join="outer" and compat="broadcast_equals".

[shoyer]

The fact that we no tests need to be changed with these new defaults is a little suspicious. It suggests that either we lack test coverage or these defaults aren't actually used in the update() codepath, which uses the internal priority_arg argument.

As discussed in #10585 (comment), a better clean-up might be to get rid of priority_arg?