[Pitch] Bump Python's required version from 3.9 to 3.11

[compnerd (Saleem Abdulrasool)]

@Adrian_Prantl

For completeness, I just wanted to note that historically our (swift.org) stable branches are not necessarily in sync with any LLVM stable branches.

Actually, we did switch to tracking stable points for the rebranch. The next sync point is release/21.x.