Tracing should only be available to Reporter users

What does this MR do and why?

• Require role >= Reporter to access Tracing
• Removed tracing_enabled? wrapper from Observability lib and relying on policies directly. That seems consistent with the rest of the codebase.

Fixes gitlab-org/opstrace/opstrace#2373 (closed)

Screenshots or screen recordings

• Before

• After

How to set up and validate locally

• Enable FF :observability_tracing
• Logout from your local GDK or go ingognito
• Visit a public project and confirm that Monitor > Tracing is not visible
• Try and visit directly https://local.gitlab.com:3443/MY-PUBLIC-PROJECT/-/tracing and confirm it's returning 404
• Log back in, and confirm the opposite, i.e. Monitor > Tracing is visible and https://local.gitlab.com:3443/MY-PUBLIC-PROJECT/-/tracing works ( might still return an error dialog as the full stack wouldn't be running )

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.