GitLab Runner: Bump default image to ubuntu:22.04

What does this MR do?

Bumps the default image from ubuntu:18.04 to ubuntu:22.04 because Ubuntu 18.04 LTS End of Standard Support is set for 2023-04-30, which is about 10 weeks away.

Removes the default Runner image, taking in the default from the Runner chart.

Also removes the Runner configuration documentation from this project, instead referring to the Runner's documentation.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Covered by existing tests.