fix: remap v1alpha2 field paths in instance group set/unset commands

[carterpewpew]

What this PR does / why we need it:

kops edit instancegroup --set spec.rootVolumeType=gp3 fails with "field not found" because the internal struct uses the nested path spec.rootVolume.type. Users naturally copy field names from the YAML they see in kops edit instancegroup, which uses the v1alpha2 flat names. This adds a field mapping that translates the seven legacy rootVolume* field names to their internal rootVolume.* equivalents in both --set and --unset commands.

Which issue(s) this PR fixes:

Fixes #17622

Special notes for your reviewer:

This mirrors how SetClusterFields already remaps legacy cluster field paths via InternalPathForClusterField.

[k8s-ci-robot]

Hi @carterpewpew. Thanks for your PR.

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