decl_module! macro: use 'frame_system' instead of system as default ident (#6500)

* Use frame_system as default ident.

* Remove unused 'frame_system' to 'system' renaming.

* Fix construct_runtime_ui tests.

* Rename system to frame_system in sudo/utility pallet test.

* Bump runtime impl_version.

* Update formatting.

substrate/frame/support/test/tests/construct_runtime_ui/missing_system_module.stderr

@@ -1,4 +1,4 @@
-error: `System` module declaration is missing. Please add this line: `System: system::{Module, Call, Storage, Config, Event<T>},`
+error: `System` module declaration is missing. Please add this line: `System: frame_system::{Module, Call, Storage, Config, Event<T>},`
  --> $DIR/missing_system_module.rs:8:2
   |
 8 |       {