[diff.class] int bit-fields now must be signed in C2Y

[Halalaluyafail3]

According to C Issue 1007 the proposal N3902 was accepted into C2Y. This would mean that the divergence between C and C++ described in [diff.class]/3 no longer is true. Should this text be updated accordingly? It appears that it was not retroactively applied to C23, though.

[jensmaurer]

C++26 and beyond is currently based on C23. We will need to update this once we rebase on C2Y, which is likely going to be post-C++29.