pgsql: Rename cluster.c to repack.c (and corresponding .h)

Rename cluster.c to repack.c (and corresponding .h)

CLUSTER is no longer the favored way to invoke this functionality, and
the code is about to shift its focus to the REPACK more ambitiously.
Rename the file to avoid leaving an unnecessary historical artifact
around.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/202603271635.owyhm7btgoic@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c0b53ec06309f955455c7d71da277991d0da4ec0

Modified Files
--------------
src/backend/commands/Makefile                |  2 +-
src/backend/commands/matview.c               |  2 +-
src/backend/commands/meson.build             |  2 +-
src/backend/commands/{cluster.c => repack.c} |  6 +++---
src/backend/commands/tablecmds.c             |  2 +-
src/backend/commands/vacuum.c                |  6 +++---
src/backend/storage/ipc/procsignal.c         |  1 +
src/backend/tcop/postgres.c                  |  1 +
src/backend/tcop/utility.c                   |  2 +-
src/include/commands/{cluster.h => repack.h} | 12 ++++++------
10 files changed, 19 insertions(+), 17 deletions(-)