pgsql: For subscription DDL, demote user mapping checks to WARNING.

For subscription DDL, demote user mapping checks to WARNING.

The checks are useful to report to the user, but there's no reason to
raise an error. If needed while constructing conninfo, fdwconnection
will raise an error then.

Partially addresses finding 1, and addresses finding 13 in report from
the linked discussion.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260710195902.4f.noahmisch%40microsoft.com
Discussion: https://postgr.es/m/e103ae8daf74485e0c0ebde297fae735d38f54d1.camel@j-davis.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/caadb9e97f571e19f359a42ad186df877ea834c7

Modified Files
--------------
src/backend/commands/subscriptioncmds.c | 8 ++++----
src/backend/foreign/foreign.c | 14 +++++++++++++-
src/include/foreign/foreign.h | 1 +
src/test/regress/expected/subscription.out | 8 +++-----
src/test/regress/sql/subscription.sql | 5 +----
5 files changed, 22 insertions(+), 14 deletions(-)