Upgrade ClusterAPI to `v1.12`

[simcod]

Description

Upgrade to ClusterAPI v1.12.
Changes to be considered.

With v1.12.0 metal-stack is available as infrastructure provider in clusterctl.

[vknabel]

Additional information regarding this upgrade:

• in Cluster API v1.11 #96 we added a few references to PRs that landed in v1.12.0
• metal-stack is now officially referenced in v1.12.0, documentation needs to be updated, a constant needs to be removed