Add deprecation notices on whole APIs

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The following APIs have some support within client-go and have been fully deprecated by GitLab for removal at some point:

• https://docs.gitlab.com/api/epic_issues/
• https://docs.gitlab.com/api/epics/
• https://docs.gitlab.com/api/geo_nodes/
• https://docs.gitlab.com/api/group_clusters/
• https://docs.gitlab.com/api/instance_clusters/
• https://docs.gitlab.com/api/project_clusters/
• https://docs.gitlab.com/api/project_vulnerabilities/

The following APIs don't appear to exist anymore but added deprecation notices anyway:

• https://docs.gitlab.com/ee/api/managed_licenses.html
• https://docs.gitlab.com/ee/workflow/time_tracking.html