Database-level collation version tracking

This patch adds to database objects the same version tracking that
collation objects have. There is a new pg_database column
datcollversion that stores the version, a new function
pg_database_collation_actual_version() to get the version from the
operating system, and a new subcommand ALTER DATABASE ... REFRESH
COLLATION VERSION.

This was not originally added together with pg_collation.collversion,
since originally version tracking was only supported for ICU, and ICU on
a database-level is not currently supported. But we now have version
tracking for glibc (since PG13), FreeBSD (since PG14), and Windows
(since PG13), so this is useful to have now. And of course ICU on
database-level is being worked on at the moment as well.

This patch is pretty much complete AFAICT. One bonus thing would be to
add a query to the ALTER DATABASE ref page similar to the one on the
ALTER COLLATION ref page that identifies the objects that are affected
by outdated collations. The database version of that might just show
all collation-using objects or something like that. Suggestions welcome.

Also, one curious behavior is that if you get to a situation where the
collation version is mismatched, every time an autovacuum worker
launches you get the collation version mismatch warning in the log.
Maybe that's actually correct, but maybe we want to dial it down a bit
for non-interactive sessions.