Add `edgedb list branches` as the branch analog to `edgedb list databases`

[raddevon]

list databases does seem to work on v5 instances, but people may not know to use it since we are re-training them that databases are out and branches are in. Looks to me like list branches could just be an alias for list databases except that I guess it should probably fail on pre-v5.

[1st1]

I think we should instead deprecate list databases and print a recommendation to use branch list

[raddevon]

Would we then make that command (edgedb branch list) work for users on versions prior to 5 to list their databases, or would that recommendation not apply to the users who would have wanted to use edgedb list databases to list databases on a pre-v5 database? Either of those seems like awkward DX to me, but maybe there's a third option I'm not considering…

[quinchs]

If you run list databases on a 5.x server we can print a deprecation warning~ but I agree that pre-5.0 should not