Remove Duo Workflow connection metrics from workhorse

What does this MR do and why?

Remove Duo Workflow connection metrics from workhorse

The counters gitlab_workhorse_duo_workflow_connections_total and gitlab_workhorse_duo_workflow_connection_errors_total are removed, along with the README sections and tests covering them.

Structured error logging is preserved; the helper that both counted and logged is renamed to logConnectionError. The error type constants (quota_exceeded, locked, other) are retained because they also drive HTTP status code mapping for the server-side execution endpoint and appear in log fields.

Connection failures are now visible only through logs. gitlab_workhorse_duo_workflow_session_errors_total remains, but only covers failures after a gRPC stream is opened, so WebSocket upgrade, request body and runner initialization failures have no metric coverage. The connections_open gauge and the HTTP action metrics are unchanged.

Changelog: removed

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