External agents should remain experiment status in 18.7

Context

MR !215064 (merged) added a feature flag to hide beta tags for sessions, triggers, agents, and flows. However, external agents should remain in Experiment status, not Beta.

What needs to be done

Ensure external agents remain experiment status before the 18.7 code cutoff.

References

• Related MR: !215064 (merged) (Feat: Add FF to hide beta tags for sessions)
• External agents documentation: https://docs.gitlab.com/user/duo_agent_platform/agents/external
• Development stages support: https://docs.gitlab.com/policy/development_stages_support/#experiment