Complete CAPD => CAPDev transition

[sbueringer]

A while ago we started the transition from CAPD to CAPDev, which was outlined in this proposal: https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20250124-From%20CAPD(docker)%20to%20CAPD(dev)%20.md

Completed:

• DevCluster & DevMachine with docker & in-memory backends: ⚠️ CAPD DevCluster and Machine #11819

Open tasks:

• DevMachinePool with docker backend
• Clearly deprecate DockerMachine / DockerCluster / DockerMachinePool CRDs (they were already deprecated, we just didn't deprecate the CRDs & Go types afaik)
• Migrate all E2E tests to DevCluster, DevMachine and DevMachinePool (with Docker backend) (includes e2e tests & YAMLs + probably test extension)
  • All e2e tests except clusterctl upgrade (should be straightforward)
  • clusterctl upgrade (let's discuss once we get here, we either have to backport DevMachinePool or drop MachinePool from these tests)
• Consider migrating some E2E tests from the docker to the in-memory backend to speed up e2e tests (TBD with maintainers which)

At a later stage: (timing TBD)

• Remove DockerCluster, DockerMachine and DockerMachinePool

[sbueringer]

/assign @Karthik-K-N

(as discussed, thx for volunteering for this)