pgsql: Rename column slotsync_skip_at to slotsync_last_skip.

Rename column slotsync_skip_at to slotsync_last_skip.

Commit 76b78721ca introduced two new columns in pg_stat_replication_slots
to improve monitoring of slot synchronization. One of these columns was
named slotsync_skip_at, which is inconsistent with the naming convention
used for similar columns in other system views.

Columns that store timestamps of the most recent event typically use the
'last_' in the column name (e.g., last_autovacuum, checksum_last_failure).
Renaming slotsync_skip_at to slotsync_last_skip aligns with this pattern,
making the purpose of the column clearer and improving overall consistency
across the views.

Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Michael Banck <mbanck@gmx.net>
Discussion: https://postgr.es/m/20251128091552.GB13635@p46.dedyn.io;lightning.p46.dedyn.io
Discussion: https://postgr.es/m/CAE9k0PkhfKrTEAsGz4DjOhEj1nQ+hbQVfvWUxNacD38ibW3a1g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5db6a344abc2dfbc5df454cd93a096443ea4dd3e

Modified Files
--------------
contrib/test_decoding/expected/stats.out     | 12 ++++++------
doc/src/sgml/monitoring.sgml                 |  2 +-
src/backend/catalog/system_views.sql         |  2 +-
src/backend/utils/activity/pgstat_replslot.c |  2 +-
src/backend/utils/adt/pgstatfuncs.c          |  6 +++---
src/include/catalog/catversion.h             |  2 +-
src/include/catalog/pg_proc.dat              |  2 +-
src/include/pgstat.h                         |  2 +-
src/test/regress/expected/rules.out          |  4 ++--
9 files changed, 17 insertions(+), 17 deletions(-)