Remove `cilium_alert` internal API

What does this MR do and why?

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Issue: #352288 (closed)

The Network Alerts feature is being removed in %15.0. This MR removes the cilium_alert internal API endpoint which is used by the GitLab Agent in order to create alerts. Remaining network alerts code will be removed after Network Policies (#352285 (closed)), which are dependent on it.

• The feature documentation has already been removed via !85941 (merged)
• The client code is being removed via gitlab-org/cluster-integration/gitlab-agent!643 (merged)

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