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/message/test-mu-message.cc

@@ -1020,8 +1020,8 @@ abc
 	const auto id2{m2->message_id()};
 	const auto id3{m3->message_id()};
 
-	g_assert_true(g_str_has_suffix(id2.c_str(), "@mu.id"));
-	g_assert_true(g_str_has_suffix(id3.c_str(), "@mu.id"));
+	g_assert_true(id2.ends_with("@mu.id"));
+	g_assert_true(id3.ends_with("@mu.id"));
 }