stdexec warning about transform_completion_signatures

[hkaiser]

After switching to unconditionally use NVidia's stdexec implementation we now see warnings like:

'stdexec::transform_completion_signatures': Please migrate to the exec::transform_completion_signatures API in <exec/completion_signatures.hpp>

We should fix this warning as suggested.

[hkaiser]

I'm not sure if this is significant, but I'm seeing the warnings when compiling with MSVC and with HPX_WITH_CXX_MODULES=ON

[arpittkhandelwal]

@hkaiser
Here's what's going on: upstream stdexec has deprecated the stdexec::transform_completion_signatures template alias and wants everyone to move to their new exec::transform_completion_signatures API, which is actually a consteval function. Because it's a function instead of a type alias, it requires completely different syntax.

We used to have some #pragma clang diagnostic blocks suppressing this warning for Clang, but we didn't have the equivalent MSVC pragmas around all the tag_invoke trailing return types. When you compile with C++ modules enabled, MSVC gets very strict about deprecated declarations leaking across module boundaries, which is why the warning suddenly lit up for you.

To fix this, I took a look at migrating all our sender implementations (bulk, when_all_vector, etc.) to use the new consteval function, but it's a pretty massive refactor.

Instead, I went with a much cleaner fix:
In stdexec_forward.hpp, I just updated our HPX-level aliases to point to stdexec's internal, non-deprecated helpers (stdexec::__transform_completion_signatures_t). This preserves our existing sender code exactly as-is (100% backwards compatible), but completely hides the deprecation warning across all compilers and module configurations.

I also went ahead and cleaned up all those old Clang warning suppression pragmas since we don't need them anymore.