MM-48345: Fix Critical level logging (#21952)

Critical wasn't added in the StdLevels slice
and due to that mlog.Critical wasn't printing anything.
This was a complete blindspot that was missed.

We have removed all references to mlog.Critical in the codebase
and pointed Critical to be Fatal in the library.

In v8, we will remove Critical altogether.

```release-note
Servers with an encrypted key will throw an error
during startup now.
```

app/platform/metrics.go

@@ -111,7 +111,7 @@ func (pm *platformMetrics) startMetricsServer() error {
 	go func() {
 		close(notify)
 		if err := pm.server.Serve(l); err != nil && err != http.ErrServerClosed {
-			pm.logger.Critical(err.Error())
+			pm.logger.Fatal(err.Error())
 		}
 	}()