pgsql: When changing owner of a subscription, do not throw an error.

When changing owner of a subscription, do not throw an error.

Errors will be caught when the connection is actually used.

Restore uses multiple DDL commands to restore a subscription, so
checks of the intermediate state risk restore errors. In the future we
could address this with a more careful restoration order, but the
DDL-time errors are merely for convenience.

Addresses finding 2 in the report from the linked discussion.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Shlok Kyal <shlok(dot)kyal(dot)oss(at)gmail(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/a1e39c501d7a89a01c2738fecf7b66927e4e8bb7

Modified Files
--------------
src/backend/commands/subscriptioncmds.c | 24 +++++++-----------------
src/test/regress/expected/subscription.out | 5 +++++
src/test/regress/sql/subscription.sql | 4 ++++
3 files changed, 16 insertions(+), 17 deletions(-)