Doc landing page has outdated support matrix

[h-vetinari]

https://grpc.io/docs/ hasn't been updated since 2021 according to its autogenerated footer; meanwhile, grpc has moved on in many ways, e.g. grpc/grpc@f122d24 for the macOS lower bound, raising the minimum python version, etc.

[Sangamesh1997]

CC @sreenithi Can you please look into this