Temporarily disabling dashboards/manage observability menu/routes

What does this MR do and why?

Closes gitlab-org/opstrace/opstrace-ui#176 (closed)

• Temporarily disabling dashboards/manage observability menu/routes as they are not needed for our initial dogfooding release (feature is still behind feature flag ). I've only commented out the entry points ( i.e. menu items and routes ), but happy to remove it if we don't want commented out code around. Those items should be re-enabled soon.
• Renaming Explore to Explore Telemetry Data as requested by Product (#385344 (closed))

Screenshots or screen recordings

How to set up and validate locally

• https://gitlab.com/gitlab-org/opstrace/opstrace-ui/-/blob/main/contribute/developer-guide.md#run-gitlab-observability-ui-with-gdk
• (alternatively https://gitlab.com/gitlab-org/opstrace/devvm#update-the-goui-image)
• Run Rspec