pgsql: Unify some more messages

Unify some more messages

No backpatch here because of message wording changes.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/202512081537.ahw5gwoencou@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d0d0ba6cf66c4043501f6f7bf508ad920645e415

Modified Files
--------------
src/backend/commands/matview.c        | 4 ++--
src/backend/postmaster/checkpointer.c | 3 ++-
src/test/regress/expected/matview.out | 2 +-
src/test/regress/expected/stats.out   | 2 +-
4 files changed, 6 insertions(+), 5 deletions(-)