Complete CAPD => CAPDev transition

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:
* [x] DevCluster & DevMachine with docker & in-memory backends: https://github.com/kubernetes-sigs/cluster-api/pull/11819

Open tasks:
* [x] DevMachinePool with docker backend: https://github.com/kubernetes-sigs/cluster-api/pull/13346
* [x] Clearly deprecate DockerMachine / DockerCluster / DockerMachinePool CRDs: https://github.com/kubernetes-sigs/cluster-api/pull/13557
* [x] Migrate all E2E tests to DevCluster, DevMachine and DevMachinePool (with Docker backend)
* [x] Migrate all e2e test YAMLs (drop MachinePool from some clusterctl upgrade tests): https://github.com/kubernetes-sigs/cluster-api/pull/13647
* [x] Migrate test extension to use Dev* instead of Docker*: https://github.com/kubernetes-sigs/cluster-api/pull/13716
* [x] Also migrate `test/infrastructure/docker/templates` to Dev* resources: https://github.com/kubernetes-sigs/cluster-api/pull/13741
* [x] Consider migrating some E2E tests from the docker to the in-memory backend to speed up e2e tests (TBD with maintainers which):
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13764
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13806
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13866
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13885
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13895
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13898 (now possible as the in-memory backend requries less resources)

v1.15:
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/14000
* [ ] Migrate DevMachinePool to v1beta2 contract
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/14099
* [ ] https://github.com/kubernetes-sigs/cluster-api/pull/14030