[2024-03 LWG Motion 3] Remove Deprecated strstreams From C++26

[jensmaurer]

https://wiki.edg.com/pub/Wg21tokyo2024/StrawPolls/P2867R2.html

[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.