Doc landing page has outdated support matrix

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. https://github.com/grpc/grpc/commit/f122d248443c81592e748da1adb240cbf0a0231c for the macOS lower bound, raising the minimum python version, etc.

Start at https://grpc.io/docs/ and inspect the landing page's support matrix and autogenerated footer. Compare the listed platform and Python versions with the changes referenced in commit f122d248443c81592e748da1adb240cbf0a0231c and current project support information; done means the matrix reflects supported versions and platforms.

Written by the indexing model from the issue text.