Issue #3586599 by yas: Refactor deprecated 'file_validate_extensions' to use the 'FileExtension' constraint instead in OpenStackStackCreateForm

Yas Naoirequested to merge
issue/cloud-3586599:3586599-refactor-deprecated-filevalidateextensions into 8.x

Issue #3586599 by yas: Refactor deprecated 'file_validate_extensions' to use the 'FileExtension' constraint instead in OpenStackStackCreateForm

Closes #3586599

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