Remove deprecated Managed-Cluster-Applications.gitlab-ci.yml
### Background
`Managed-Cluster-Applications.gitlab-ci.yml` was **deprecated** in https://gitlab.com/gitlab-org/gitlab/-/issues/327908, 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](https://docs.gitlab.com/ee/user/clusters/management_project_template.html) 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`.
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