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-utils-file.cc

@@ -201,7 +201,7 @@ Mu::read_from_stdin()
  */
 /*LCOV_EXCL_START*/
 static void
-maybe_setsid (G_GNUC_UNUSED gpointer user_data)
+maybe_setsid ([[maybe_unused]] gpointer user_data)
 {
 #if HAVE_SETSID
 	setsid();