release-19.2: opt: backport two pg compatibility fixes for GROUP BY

[jseldess]

PR: cockroachdb/cockroach#42694

From release notes:

It is now valid for SELECT and HAVING to refer to ungrouped columns in the special case when the grouped columns contain the primary key of the table containing the ungrouped column. [#42694][#42694] {% comment %}doc{% endcomment %}

It is now supported to specify selection target aliases as GROUP BY columns. Note that the FROM columns take precedence over the aliases, which are only used if there is no column with that name in the current scope. [#42694][#42694] {% comment %}doc{% endcomment %}

[yuzefovich]

I was just casually strolling through known limitations page and saw this limitation which has been fixed by Radu recently, so I decided to drop a friendly reminder to update the page when this issue is addressed for 19.2 and 20.1 docs.

[ericharmeling]

Let's consolidate this issue with #6181.