[CI] Remove OCP 4.8 jobs

What does this MR do?

[CI] Remove OCP 4.8 jobs

Removes jobs that test against OCP 4.8 given the cluster cleanup done in gitlab-org/distribution/team-tasks#1202 (closed).

Related issues

Closes #1032 (closed)

Test plan

1. See https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/pipelines/805875657.
2. Ensure no 4.8 jobs are present.

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
• Delete KUBECONFIG_OCP_4_8 CI variable
• After merge, pick into latest stable branch

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.