Remove GKE 1.25 and 1.26 CI jobs

What does this MR do?

Removes pipeline jobs for Kubernetes 1.25 and 1.26 clusters that are being decommissioned as part of issue Support deploying GitLab on Kubernetes 1.29 (gitlab-org/distribution/team-tasks#1473 - closed).

Docs update MR Deprecate support for Kubernetes 1.25 and 1.26 (!857 - merged) should be merged before this change.

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.

Related issues

Closes #1690 (closed)