🐛 Fix KCP maxSurge field to be mutable

[jan-est]

This PR is fixing issue 4437. Webhook defaulting in v0.3.15 causing an error during KCP upgrade after upgrading cluster from v0.3.x to v0.3.15. This PR is changing every field under spec.rolloutStrategy mutable.

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 #

[k8s-ci-robot]

Hi @jan-est. 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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[randomvariable]

/ok-to-test

[randomvariable]

We probably also want to include the test case here as well.

[jan-est]

Would this patch be merged to next release v0.3.16?

[fabriziopandini]

/lgtm
/approve

Would this patch be merged to next release v0.3.16?

yes

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini

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