Switch to ssa.Patch to create machines in machinepool controller

[CecileRobertMichon]

I would recommend using the following here:

if err := ssa.Patch(ctx, r.Client, MachinePoolControllerName, machine); err != nil {

Otherwise you end up with co-ownership of fields between "manager" and "machinepool-controller" like here: https://storage.googleapis.com/kubernetes-jenkins/logs/periodic-cluster-api-e2e-main/1725100075312484352/artifacts/clusters/bootstrap/resources/quick-start-jwp00u/Machine/quick-start-loyh5w-mp-0-9svll-hk7jp.yaml

The consequence is that it's impossible to unset fields via "machinepool-controller"

_Originally posted by @sbueringer in https://github.com/kubernetes-sigs/cluster-api/pull/9619#discussion_r1395699740_

[CecileRobertMichon]

Following discussion https://github.com/kubernetes-sigs/cluster-api/pull/9725#discussion_r1399668133:

fake client doesn't support SSA. Sorry didn't think about that when recommending using SSA.

For reference. In KCP we previously had a flag to skip some optional SSA part until we migrated the tests over to envtest (https://github.com/kubernetes-sigs/cluster-api/issues/8393).

But I think we should aim for using SSA consistently before the v1.6.0 release. Otherwise we end up with strange issues in our release.

The ideal solution for this now is to migrate the affected tests to envtest.

This will require migrating tests to use envtest

[sbueringer]

/triage accepted

[CecileRobertMichon]

/assign