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-regex.hh

@@ -178,7 +178,7 @@ private:
 	GRegex *rx_{};
 };
 
-static inline std::string format_as(const Regex& rx) {
+inline std::string format_as(const Regex& rx) {
 	if (auto&& grx{rx.g_regex()}; !grx)
 		return "//";
 	else