modernization: tweaks for c++20

Update various places for what we can do with C++20:

- ends_with / starts_with
- std::to_array
- using instead of typedef
- designated initializers
- "[[maybe_unused]]" instead of G_GNUC_UNUSED

Also remove some unnecessary 'static'

lib/message/mu-message.hh

@@ -512,7 +512,7 @@ private:
 }; // Message
 MU_ENABLE_BITOPS(Message::Options);
 
-static inline auto
+inline auto
 format_as(const Message& msg) {
 	return msg.path();
 }