switch gui_app deprecation to FeatureDeprecatedKwargs

The deprecation message for "gui_app" is appearing for every target
rather than just once, and even if the required version is older
than 0.56.0.  Use @FeatureDeprecatedKwargs to fix both issues.

mesonbuild/build.py

@@ -929,7 +929,6 @@ This will become a hard error in a future Meson release.''')
             # was not specified and win_subsystem should be used instead.
             self.gui_app = None
             if 'gui_app' in kwargs:
-                mlog.deprecation('The gui_app kwarg is deprecated, use win_subsystem instead.')
                 if 'win_subsystem' in kwargs:
                     raise InvalidArguments('Can specify only gui_app or win_subsystem for a target, not both.')
                 self.gui_app = kwargs['gui_app']

mesonbuild/interpreter.py

@@ -3874,6 +3874,7 @@ external dependencies (including libraries) must go to "dependencies".''')
 
     @FeatureNewKwargs('executable', '0.42.0', ['implib'])
     @permittedKwargs(permitted_kwargs['executable'])
+    @FeatureDeprecatedKwargs('executable', '0.56.0', ['gui_app'], extra_message="Use 'win_subsystem' instead.")
     def func_executable(self, node, args, kwargs):
         return self.build_target(node, args, kwargs, ExecutableHolder)