pgsql: Reword recent error messages: "should" -> "must"

Reword recent error messages: "should" -> "must"

Most were introduced in the 17 timeframe. The ones in wparser_def.c are
very old.

I also changed "JSON path expression for column \"%s\" should return
single item without wrapper" to "JSON path expression for column \"%s\"
must return single item when no wrapper is requested" to avoid
ambiguity.

Backpatch to 17.

Crickets: https://postgr.es/m/202501131819.26ors7oouafu@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db19a5061ce954320f47a65c169081cbb2d920f8

Modified Files
--------------
src/backend/tsearch/wparser_def.c | 8 ++++----
src/backend/utils/adt/jsonpath_exec.c | 12 ++++++------
src/test/regress/expected/sqljson_jsontable.out | 2 +-
src/test/regress/expected/sqljson_queryfuncs.out | 8 ++++----
4 files changed, 15 insertions(+), 15 deletions(-)