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/mu-query-threads.cc

@@ -637,7 +637,7 @@ struct MockQueryResult {
 
 using MockQueryResults = std::vector<MockQueryResult>;
 
-G_GNUC_UNUSED static std::ostream&
+[[maybe_unused]] static std::ostream&
 operator<<(std::ostream& os, const MockQueryResults& qrs)
 {
 	for (auto&& mi : qrs)