Change the silence manager to work with the new alert manager

[Rotfuks]

Motivation

We need to make sure that alerts managed through the mimir alertmanager can be silenced the same way as the alerts in the prometheus alerts manager. For this we need to potentially change something in the silence manager.

Todo

• We want to load our silences into the giantswarm tenant, so we need to work on that.
• There are some PRs open for this, we need to review them if that’s enough

Outcome

• The silence manager works with the new mimir alert manager

[QuentinBisson]

PR giantswarm/silence-operator#318

[QuentinBisson]

Waiting for feedback on my pr