dev docs: back to PostgreSQL 14 for NixOS dev VM

PG 15 changed the default permission for the public schema
which isn't compatible with NixOS `ensureUsers`. To upgrade
to 15, we need to add our own setup unit or wait until nixpkgs
figures out a fix for that.

docs/development/ekklesia_dev.nix

@@ -129,7 +129,7 @@ in
         };
       }
     ];
-    package = pkgs.postgresql_15;
+    package = pkgs.postgresql_14;
   };
 
   users.defaultUserShell = pkgs.zsh;