pgsql: First-draft release notes for 16.1.

First-draft release notes for 16.1.

As usual, the release notes for other branches will be made by cutting
these down, but put them up for community review first.

Also as usual for a .1 release, there are some entries here that
are not really relevant for v16 because they already appeared in 16.0.
Those'll be removed later.

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1e774846b870a858f8eb88b3f512562009177f45

Modified Files
--------------
doc/src/sgml/release-16.sgml | 1568 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 1562 insertions(+), 6 deletions(-)