[2024-03 LWG Motion 5] Remove wstring_convert From C++26

[jensmaurer]

https://wiki.edg.com/pub/Wg21tokyo2024/StrawPolls/P2872R3.pdf

[StephanTLavavej]

Noticed a typo:

Change: Remove convenience interfaces wstring_buffer and wbuffer_convert.

should say:

Change: Remove convenience interfaces wstring_convert and wbuffer_convert.

[AlisdairM]

Could this issue be assigned to me? I don't see the UI to assign issues.

[AlisdairM]

This PR has been consolidated into #6921 that resolves merge conflicts among the 4 deprecation removal papers.