Fix VolumeSubpathEnvExpansion feature gate

[sftim]

Move the VolumeSubpathEnvExpansion feature gate information out of the Feature gates for Alpha or Beta features section.

@daminisatya: this is a fixup for #16547

/kind cleanup
/milestone 1.17

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In response to this:

Move the VolumeSubpathEnvExpansion feature gate information out of the Feature gates for Alpha or Beta features section.

@daminisatya: this is a fixup for #16547

/kind cleanup
/milestone 1.17

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[kbhawkey]

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/lgtm

[daminisatya]

Thank you @sftim

/lgtm
/approve

[k8s-ci-robot]

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This pull-request has been approved by: daminisatya

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