[clang] [Clang] Warn with -Wpre-c23-compat instead of -Wpre-c++17-compat for u8 character literals in C23 (PR #97210)

[via cfe-commits]

cor3ntin wrote:

It's a bit weird to have an extension warning given we seem to not support u8 in older language modes at all
https://www.godbolt.org/z/1KrqGeT1q 

I think we should support the extension before in addition of having a warning for it
@AaronBallman 

https://github.com/llvm/llvm-project/pull/97210