module: increase priority for posgresql package default

This gives the default value from this module a slightly higher
priority than the upstream module's default, while still allowing users
to simply set `services.postgresql.package` using the default priority.

The change in 8bc790171f introduced
`mkDefault` for the postgresql package.
Unfortunately the upstream package option default is also specified
using `mkDefault` instead of the more appropriate `mkOptionDefault`.

This meant that users with a `system.stateVersion` other than `22.05`,
`22.11` or `23.05` got an evaluation error because there are two
conflicting definitions for the package option.

module.nix

@@ -19,7 +19,8 @@ let
   inherit (lib.modules)
     mkDefault
     mkIf
-    mkMerge;
+    mkMerge
+    mkOverride;
 
   inherit (lib.options)
     mdDoc
@@ -354,7 +355,12 @@ in
     #
     # After postgresql_14 has been removed from nixpkgs, this workaround can be dropped.
     (mkIf (versionOlder config.system.stateVersion "24.05") {
-      services.postgresql.package = lib.mkDefault pkgs.postgresql_14;
+      # The upstream postgresl module is using mkDefault
+      # to specify the default value for the package option.
+      # Unfortunately this forces us to specify this default with
+      # a higher priority, i.e. lower number, than mkDefault which
+      # has priority 1000
+      services.postgresql.package = mkOverride 999 pkgs.postgresql_14;
     })
   ];
 }