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/utils/mu-error.hh

@@ -183,7 +183,7 @@ private:
 	std::string		hint_;
 };
 
-static inline auto
+inline auto
 format_as(const Error& err) {
 	return mu_format("<{} ({}:{})>",
 			 err.what(),