Add types to StatefulSetUpdateStrategyType constants

[abursavich]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This adds types to StatefulSetUpdateStrategyType constants, which are currently untyped strings.

This is an issue for me personally because I am generating code from the API packages and there isn't a clean way to tie these untyped string constants to their typed "enum".

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
As precedence, this change is very similar to #70521.

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @abursavich. Thanks for your PR.

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[fejta-bot]

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[draveness]

/kind bug
/priority backlog
/assign @liggitt

[draveness]

/ok-to-test

[liggitt]

/retest
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abursavich, liggitt

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