documentation updates for SQL:2023

SQL:2023 should be published within the next 2 months, so I want to 
update our SQL conformance information for our PostgreSQL release later 
this year.

Attached are patches that update the keywords list and the features list 
as usual.  (Some of the new features in the JSON area are still being 
worked on.  I have just set them all to NO for now, to be revisited later.)

I'm also proposing to get rid of the tracking of subfeatures.  This has 
been de-facto deprecated: All the subfeatures for optional features have 
been removed (replaced by top-level feature codes), and the subfeatures 
for mandatory features aren't very interesting.  The TODO is to remove 
the columns for the subfeatures in src/backend/catalog/sql_features.txt. 
  That is a mechanical change that I did not include in the patch.

I'll leave this patch set in the commit fest, to let those concurrent 
developments shake out and as a reminder to address this when the time 
comes.