✨ Mark specific KCP machines with delete annotation for scaling down

[furkatgofurov7]

What this PR does / why we need it:

Which issue(s) this PR fixes:
Adds a possibility to mark a specific control plane Machine to be deleted by a KCP when one or multiple machines are annotated by delete-machine annotation.

This PR:

1. makes sure the oldest machine is deleted first when there is one or more machine is marked for deletion. It implements the prioritization logic in descending order with respect to rollout policies, while selecting machine for scale down in the following order:
   • outdatedMachines with the delete annotation
   • machines with the delete annotation
   • outdated machines
   • all machines
2. adds a unit test.
3. updates the KCP proposal with a note describing the behavior for scale down with annotation.

Backport of #3948

[k8s-ci-robot]

Hi @furkatgofurov7. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[furkatgofurov7]

/assign @fabriziopandini
/cc @vincepri @detiber

[vincepri]

/ok-to-test
/milestone v0.3.x

[vincepri]

/lgtm
/approve

This is going to go in the next patch release

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

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[vincepri]

/retest

[furkatgofurov7]

/test pull-cluster-api-test-release-0-3

[furkatgofurov7]

@vincepri my apologies, github UI was a bit slow, so I re-triggered the test again :)