Remove observability tabs and associated code at group level

What does this MR do and why?

Removes the previous iteration of Observability that was accessible through the Group menu (Monitor nav item). This removes the navigation items, the embedded GitLab Observability UI (Grafana fork) and all associated code that is not used in the newer iteration of observability that is now available under the Monitor > Tracing menu of a Project.

This code is still under Experiment and is behind the observability_group_tab feature flag which has only been enabled for internal groups. The backend APIs have already disabled so these views currently render error messages.

This MR is on the larger side because its removing a set of features and their tests. Removing this in a single MR was deemed to be the cleanest action to validate proper removal.

Closes gitlab-org/opstrace/opstrace#2365 (closed)

Screenshots or screen recordings

Before this MR:

Explore Telemetry tab

Data Sources tab

After this MR:

Monitor navigation item (and sub items Explore Telemetry and Data Sources) are removed

Explore Telemetry route no longer exists

Data Sources route no longer exists

Project level Tracing Tab & Tracing Detail view continue to work as expected

How to set up and validate locally

• Enable the feature flag observability_group_tab globally and navigate to a group in GitLab.
• Verify that the Monitor menu does not contain Explore telemetry data or Data sources.
• Verify that the following routes result in 404:
  • gdk.local/[any-gitlab-group-path]/-/observability/explore
  • gdk.local/[any-gitlab-group-path]/-/observability/datasources

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.