<experimental/generator> should be deprecated, then removed

[StephanTLavavej]

We've merged C++23 <generator> for VS 2022 17.13 Preview 1 (#4953).

It's time to deprecate and then remove <experimental/generator>, since we don't want to encourage users writing non-Standard code when superior replacements are available. (Additionally, early adopters of experimental machinery should know what they're signing up for and can reasonably be expected to be eager upgraders.) This will also avoid bugs in the experimental implementation like #4999.

According to @CaseyCarter, <experimental/generator> generally has very similar behavior to <generator> except in weird corner cases.

For removal, we do need to investigate whether Windows took a dependency on <experimental/generator> in kernel mode, which <generator> does not (yet) attempt to support. If we had to support kernel mode in this one header, it would have to become a core header and we would have to change how we verify preconditions:

STL/stl/inc/generator

Line 480 in faccf00

_STL_VERIFY(!_Coro.done(), "Can't dereference generator end iterator");

Deprecation in 17.13 and removal in 18.0 sounds like a good plan to us.

[frederick-vs-ja]

<experimental/generator> is not a core header, but it seems that we're just making it usable in kernel mode by skipping somethings. It should be investigated what are necessary for skipping.
It's a bit surprising me that <experimental/generator> includes the whole <memory>...