Remove deprecated Managed-Cluster-Applications.gitlab-ci.yml

Background

Managed-Cluster-Applications.gitlab-ci.yml was deprecated in #327908 (closed), but the template can still be included via include:template, and CI/CD managed apps are still documented.

Proposal

By GitLab %15.0, the Managed-Cluster-Applications.gitlab-ci.yml any remaining documentation of the feature should be removed.

How does this affect users?

This will be a breaking change for users of include:template for Managed-Cluster-Applications.gitlab-ci.yml (new pipelines will not find the file). Others will be unaffected.

Migrating away from include:template for Managed-Cluster-Applications.gitlab-ci.yml

• Users who wish to upgrade to Helm v3 or gain more control over their releases can use the new project template to get started.
• Users who are not ready to migrate can stop using include:template, and instead inline the template's contents in their .gitlab-ci.yml.