Datasource: Error when adding Mimir alert manager data source

### What happened? Hi, We can't get Mimir alert manager data source to work. We run self-hosted Grafana 11.4.0, Mimir, Loki Tempo on a single AKS Kubernetes 1.29.4 cluster. When trying to add Mimir alert manager data source we get `It looks like you have chosen a Mimir or Cortex implementation, but detected a Prometheus endpoint. Please update implementation selection and try again.` message even though it is the correct Mimir alert manager url (http://mimir-nginx.observability.svc:80/alertmanager). When `Implementation` field changed to Prometheus then health check works but we still don't see alert manager in Alerting Rules page as a data source. Grafana: 11.4.0 Mimir: 2.15.0  ### What did you expect to happen? Mimir alert manager works. ### Did this work before? We are replacing Prometheus with Mimir so we haven't run it before. ### How do we reproduce it? 1. Deploy Grafana 11.4.0 on AKS Kubernetes 1.29.0 using Grafana official Helm chart 2. Deploy Mimir 2.15 default deployment mode on AKS Kubernetes 1.29.0 using Mimir official Helm chart 3. Try to configure Mimir alert manager data source on UI. ### Is the bug inside a dashboard panel? Grafana Alert Manager Data Source configuration page. ### Environment (with versions)? Grafana: 11.4.0 OS: AKSUbuntu-2204 Browser: Edge ### Grafana platform? Kubernetes ### Datasource(s)? AlertManager