Remove artifacts:reports:cluster_applications in GitLab

Problem to solve

In !63897 (merged), we removed the parser for articacts:reports:cluster_applications (because it had no user-visible impact, and relied on code being removed), but left the YAML keywords in place to avoid breaking existing pipelines. This is ~"technical debt" that needs to be cleaned up, but removing it is a breaking change.

Proposal

Remove support for the artifacts:reports:cluster_applications artifact in %15.0.

More info

At the time of writing, the feature depending on this artifact upload was deprecated in %14.0 and is scheduled for removal in %15.0,