Deploy and test the Mimir Alert Manager

[Rotfuks]

Motivation

In order to migrate to the new Mimir alertmanager we need to deploy and test an insdtance of the alert manager first.

Todo

• [x] Deploy and test the mimir alertmanager on a test installation
  • [x] Configure the Ruler to the new Alert Manager
  • [ ] Configure the notification template

Outcome

• We know how to deploy the mimir alertmanager in the right way

[QuantumEnigmaa]

We are currently doing tests on golem together with @TheoBrigitte . See https://github.com/giantswarm/roadmap/issues/3746#issuecomment-2497949177