Deprecate Grafana in Monitoring role

As discussed in https://gitlab.com/gitlab-org/distribution/team-tasks/-/issues/1257#note_1362883324, bundled Grafana is no longer supported in 16.0, and it will be completely removed from the package in 16.3. There is no substitution for it as GitLab Observability UI is also deprecated.

The issue is to deprecate installing Omnibus-bundled Grafana via the Monitoring role to follow GitLab guidance. To enable graceful transition period:

• Add monitor_enable_deprecated_grafana flag in GET to control whether to install the deprecated bundle anyway by applying the correct flags to the gitlab.rb file.
  • Based on principle of least surprise for existing users, currently defaulting it to TRUE to at least get us through the initial 16.0 release.
• Add deprecation note in GET release with the above update - with clarification that this only affects non hybrid envs