Fix tiers on AI audit events metric definitions

What does this MR do and why?

AI audit events are documented as available on Premium and Ultimate, but the tiers: metadata on the two count_ai_audit_events metric definitions (ee/config/metrics/counts_all/20260609210336_count_ai_audit_events.yml and 20260609210336_count_ai_audit_events_clickhouse.yml) only listed ultimate. Nothing in the write path restricts these events to an Ultimate license, so the metrics can report data from Premium instances too. This MR adds premium to both files to match the documented tiers in doc/user/duo_agent_platform/ai-audit-events.md ("Tier: Premium, Ultimate").

The files were introduced in !239886 (merged). The same fix is being applied to the equivalent unmerged metrics in !249175 (merged). Metadata-only change, no behavior impact.

References

• !239886 (merged)
• !249175 (merged)

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