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

• Deploy and test the mimir alertmanager on a test installation
  • 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 #3746 (comment)

[QuantumEnigmaa]

As @TheoBrigitte managed to make the ruler communicate with alertmanager, we are now able to deploy and use mimir-alertmanager as our main alertmanager. There is still work to do concerning the automatic loading of its config using the observability-operator but at least we can close this issue.