fix(pg_stat_monitor): use custom test instead of default for pg_upgrade compatibility

Remove pg_stat_monitor from default extension test list to prevent conflict
with the custom pg_stat_monitor.nix test file. The custom test includes
special logic to skip pg_regress after pg_upgrade when no update script
is generated, which is necessary because version 2.1 has different schemas
between PostgreSQL 15 and 17 despite sharing the same version number.

Author: yvan-sraka

nix/ext/tests/default.nix

@@ -299,7 +299,6 @@ builtins.listToAttrs (
       "pg_net"
       "pg_partman"
       "pg_repack"
-      "pg_stat_monitor"
       "pg_tle"
       "pgaudit"
       "postgis"