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-store.hh

@@ -571,7 +571,7 @@ private:
 MU_ENABLE_BITOPS(Store::Options);
 MU_ENABLE_BITOPS(Store::MoveOptions);
 
-static inline std::string
+inline std::string
 format_as(const Store& store)
 {
 	return mu_format("store ({}/{})", format_as(store.xapian_db()),