pgsql: Unify REPACK (CONCURRENTLY) error messages.

Unify REPACK (CONCURRENTLY) error messages.

Commit 86f7c82cf102 reported the user catalog table restriction as a
detail, but the other checks in check_concurrent_repack_requirements()
were still a mix: some were hints, and the details repeated
"REPACK (CONCURRENTLY)", which the primary message already names.

Report them as details with the same "This operation ..." wording. The
one actionable hint, ALTER TABLE ... REPLICA IDENTITY USING INDEX,
stays.

Backpatch to v19, where REPACK (CONCURRENTLY) was introduced.

Author: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Suggested-by: Alvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/apBIFWzHYOaG0auN%40nathan
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4964c75b0889ab79a268dc1e1027e4dabe0c7dfd

Modified Files
--------------
contrib/test_decoding/expected/repack.out | 12 ++++++------
src/backend/commands/repack.c | 22 ++++++++--------------
2 files changed, 14 insertions(+), 20 deletions(-)