[Bug libstdc++/109162] C++23 improvements to std::format

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109162
Bug 109162 depends on bug 113318, which changed state.

Bug 113318 Summary: [C++23] Implement P1185R12, Naming text encodings to demystify them
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113318

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED