pgsql: Reject sequence synchronization against pre-PostgreSQL 19 publis

Reject sequence synchronization against pre-PostgreSQL 19 publishers.

Sequence synchronization requires the page_lsn field returned by
pg_get_sequence_data(), which was added in PostgreSQL 19. Previously,
requesting sequence synchronization against an older publisher (via
ALTER SUBSCRIPTION ... REFRESH SEQUENCES or by running
ALTER SUBSCRIPTION ... CONNECTION on a disabled subscription with
sequences in the INIT state and subsequently enabling the subscription)
would cause the sequence synchronization worker to repeatedly fail with a
confusing "invalid query response" error.

Check the publisher's server version up front in both
AlterSubscription_refresh_seq() and copy_sequences(), and error out
immediately when it predates PostgreSQL 19.

Also document the PostgreSQL 19 publisher requirement for sequence
replication in the logical replication documentation and in
ALTER SUBSCRIPTION ... REFRESH SEQUENCES.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Author: vignesh C <vignesh21(at)gmail(dot)com>
Reviewed-by: Shveta Malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Backpatch-through: 19
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/57e7c52c0fe43fff36d0f4440eea13cf5668b160

Modified Files
--------------
doc/src/sgml/logical-replication.sgml | 18 +++++++++++++++++-
doc/src/sgml/ref/alter_subscription.sgml | 6 ++++++
src/backend/commands/subscriptioncmds.c | 10 ++++++++++
src/backend/replication/logical/sequencesync.c | 10 ++++++++++
4 files changed, 43 insertions(+), 1 deletion(-)