Bump advertised OpenMP version

[h-vetinari]

This already came up in conda-forge/_openmp_mutex-feedstock#10 and #83, though until now, support as advertised was clearly only 4.5

However, as of llvm/llvm-project@0c6f2f6, LLVM 17 will use OpenMP 5.1 as the default version.

This is somewhat surprising to me, because https://clang.llvm.org/docs/OpenMPSupport.html still shows much of 5.0/5.1 as unimplemented. I've asked in the MR about the status.

In any case, it's a reality that -fopenmp-version={45,50,51} can be used to select the implementation level already today, so perhaps the switch of the upstream default is a good enough reason to bump the version here?