Consider returning warnings in validation for usage of deprecated fields

[chrischdi]

What would you like to be added (User Story)?

As a user I would like to get a warning when I make use of deprecated fields.

Detailed Description

We already have some deprecated fields:

• [ ] Cluster.spec.topology.rolloutAfter
• [ ] DockerMachine.spec.bootstrapped
• [ ] ClusterClass.spec.Variables[].Metadata?
• [ ] https://github.com/kubernetes-sigs/cluster-api/issues/104790
• [ ] https://github.com/kubernetes-sigs/cluster-api/issues/10666
• [ ] https://github.com/kubernetes-sigs/cluster-api/pull/10778
• [ ] experimentalRetryJoin
• ...

and considering e.g.:

• 10708

we may introduce more in future.

For providing a user the information that it makes use of a deprecated field, we could use warnings in validation to bubble that information up.

Anything else you would like to add?

No response

Label(s) to be applied

/kind feature One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels.

[sbueringer]

Yup also

• 10479

• 10666

• 10778

• experimentalRetryJoin

I think this is a great idea, and we should do this for all fields now & going forward

[fabriziopandini]

/triage accepted /help

we can evaluate case by case on follow up PRs

[k8s-ci-robot]

@fabriziopandini: This request has been marked as needing help from a contributor.

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In response to [this](https://github.com/kubernetes-sigs/cluster-api/issues/10725): >/triage accepted >/help > >we can evaluate case by case on follow up PRs Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[chrischdi]

Yup also

• Consider deprecation of revision management in MachineDeployments #10479
• Deprecated definitionFrom in cluster variables #10666
• ✨ Add metadata for top-level and nested variables & deprecate previous metadata field #10778
• experimentalRetryJoin

I think this is a great idea, and we should do this for all fields now & going forward

moved these up to the list