Route alert notifications through single controller

What does this MR do and why?

Swaps the /prometheus/alerts/notify route to direct to Projects::Alerting::NotificationsController#create, so all alert notifications go through the same controller/auth flows.

This prepares us to ingest alerts via migrated prometheus alert integrations smoothly. No user-facing changes are expected.

• Related issue: Allow Prometheus' metrics dashboard and incomin... (#338838)
• Related prep MRs:
  • Add support for Prometheus as HTTP alert integr... (!120046 - merged)
  • Support custom mappings for Prometheus integrat... (!120055 - merged)
  • Swap to unique endpoint identifier for promethe... (!125449 - merged)

How to set up and validate locally

See !123217 (merged)

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.