Resolve remaining C++20 TODOs

[rubiefawn]

Resolves the following TODO comments:

• include/ArrayVector.h:309: TODO C++20: Use std::construct_at
• include/ArrayVector.h:365: TODO C++20: Replace with operator<=>
• include/ArrayVector.h:378: TODO C++20: Remove
• include/Flags.h:71: TODO C++20: = default
• include/Flags.h:72: TODO C++20: Remove

The TODO in include/lmms_math.h:326 TODO C++20: use std::formatted_size has some compiler support and is handled separately in #8321.

There are likely many other places in the codebase that could benefit from C++20 features. This PR is focused on resolving explicit TODO comments only.

[messmerd]

In ArrayVector.h, you can also remove the entire detail namespace and then fix the template parameters of some methods like this:

-	template<typename It, std::enable_if_t<detail::is_input_iterator_v<It>, int> = 0>
+	template<std::input_iterator It>

[messmerd]

Maybe you could also add a note about std::inplace_vector to the comment before the ArrayVector class:

/**
 * A container that stores up to a maximum of `N` elements of type `T` directly
 * within itself, rather than separately on the heap. Useful when a dynamically
 * resizeable container is needed for use in real-time code. Can be thought of
 * as a hybrid between `std::array` and `std::vector`. The interface follows
 * that of `std::vector` - see standard C++ documentation.
+ *
+ * TODO C++26: Use std::inplace_vector instead
 */