[2.8] Validate allocatable vGPUs number in Harvester clusters creation

[torchiaf]

Summary

Fixes https://github.com/rancher/dashboard/issues/10916

Occurred changes and/or fixed issues

Technical notes summary

See https://github.com/rancher/dashboard/pull/10911

Areas or cases that should be tested

Areas which could experience regressions

Screenshot/Video

Checklist

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