Update README
Update README

Replace CI badge with GitHub Actions badge. Update vSphere version
information. Remove section about support for older Ruby versions as
they are no longer supported.

Signed-off-by: J.R. Garcia <837382919d8fe61d4198de0c3cb7269cb0eb414f@vmware.com>

10xEngineer/rbvmomi,10xEngineer/rbvmomi,vmware/rbvmomi