🌱 Add OpenAPI defaulting detection for KubeadmConfig by using forbiddenmarkers

[nayuta723]

What this PR does / why we need it:
This PR introduces a new forbiddenmarkers linter check targeting the KubeadmConfigSpec struct and its nested fields. We've repeatedly encountered issues where adding new default values via OpenAPI markers (e.g., // +kubebuilder:default) leads to unintended KubeadmControlPlane (KCP) rollouts following a CAPI upgrade.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #8147

/area ci

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[sbueringer]

Let's go one step further and block all defaulting (as we don't want to use OpenAPI defaulting at all nowadays)

I think we can drop this config here and then add an exclude for existing markers. If I see correctly there is only one usage of the default marker in v1beta2 today.
Let's add this exclude in its own section directly above "# TODO: Excludes that should be removed once the corresponding issues in KAL are fixed"

(v1beta1 is probably already excluded correctly in l.91++)

[sbueringer]

/ok-to-test

[JoelSpeed]

If we add other forbidden markers in the future, this will prevent them from being picked up in the wider APIs.

Have you considered making the text more specific to only pick up the default markers?

[sbueringer]

See: #12851 (comment)

[nayuta723]

@sbueringer @JoelSpeed
Thank you for the review. I've made the corrections, so please take a look.

[sbueringer]

/lgtm
/assign @JoelSpeed

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: c7c8dd56aec60c3c6d571efc47f4604cc5153642

[sivchari]

/lgtm

[sbueringer]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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[sbueringer]

Last nits

[sbueringer]

Sorry shouldn't have approved and added comments. I'll follow-up :)

(xref: #12869)