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'

mu/tests/test-mu-query.cc

@@ -111,10 +111,10 @@ run_and_count_matches(const std::string& xpath,
 	return qres->size();
 }
 
-typedef struct {
+struct QResults {
 	const char* query;
 	size_t      count; /* expected number of matches */
-} QResults;
+};
 
 static void
 test_mu_query_01(void)