Dynamic set the AKS version for management cluster

[zylxjtu]

Hard-pinning the aks version (previously --kubernetes-version 1.33.5) for CAPZ management cluster breaks CI every time AKS retires an old minor version. Set MGMT_K8S_VERSION to pin explicitly, otherwise use the region's current default GA version, falling back to the highest available version.

[marosset]

/lgtm
/approve
/hold for CI

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: marosset, zylxjtu

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[zylxjtu]

/test pull-e2e-capz-windows-2025-extension

[kubernetes-prow]

@zylxjtu: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-e2e-capz-windows-2025-extension	6bc0dd3	link	false	/test pull-e2e-capz-windows-2025-extension

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[zylxjtu]

will need to check for the 2025 test failure, but it passed the manager cluster creating stage, so the fix did works, will check in anyway to unblock the main master test job failure

[zylxjtu]

/hold cancel