Doc: update list of supported operating systems

[Bischoff]

This PR synchronizes the documentation with the code of get_linux_dist():

        supported = {'rhel': ['6', '7', '8', '9'],
                     'sles': ['11', '12', '15'],
                     'ubuntu': ['16', '20', '22'],
                     'rhcos': ['4']}

rhel8.x, rhel9.x, sles15.x, ubuntu20.x, and ubuntu22.x were missing in the doc.

These texts are used in:

• documentation parameter image_metadata for Create image
  • documentation parameter guest_os_version for Attach volume, Detach volume, Delete network interface, and Grow root volume of guest
  • documentation parameter guest_os_version_all for Create network interface and List images

I tried to follow all code paths and it seemed to me they all call get_linux_dist(). Please double-check as I am new to Feilong's source code, and an error is always possible.