📖 Improve v1.12 migration docs

[fabriziopandini]

What this PR does / why we need it:

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 #

/area documentation

[k8s-ci-robot]

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[fabriziopandini]

this is a placeholder, #12971 is going to add the content of this PR

[fabriziopandini]

/test pull-cluster-api-verify-main

[fabriziopandini]

/test pull-cluster-api-test-main

[fabriziopandini]

/cherry-pick release-1.12

[k8s-infra-cherrypick-robot]

@fabriziopandini: once the present PR merges, I will cherry-pick it on top of release-1.12 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.12

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