Optimize GET /api/internal/kubernetes/agent_info

In the last days this REST API endpoint seems to perform poorly and has impacted our error budget:

We should investigate if there is any underlying issue or improvements that we can make

Proposal

• Remove Gitaly feature flags from all responses rails generates for kas (ALL API endpoints, not just /api/v4/internal/kubernetes/agent_info).
• Remove the kubernetes_agent_internal_api feature flag. It was enabled by default in !41045 (merged) - extracted to [Feature flag] Cleanup kubernetes_agent_interna... (#432773 - closed)