chore: Remove support for deprecated `images` configuration attribute in favor of `image_files` attribute in the export config block

[tenthirtyam]

Summary

• Removes deprecated images option that was noted to be removed over 2 years ago.
• Updated export configuration descriptions.
• Simplified manifest validation.
• Updated to use ui.Say versus ui.Message for output consistency.
• Added code comments.
• Improved output and error messaging for output consistency.

Tests

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==> vsphere-iso.linux-photon: Executing shutdown command...
==> vsphere-iso.linux-photon: Deleting Floppy drives...
==> vsphere-iso.linux-photon: Eject CD-ROM drives...
==> vsphere-iso.linux-photon: Deleting CD-ROM drives...
==> vsphere-iso.linux-photon: Exporting to Open Virtualization Format (OVF)...
==> vsphere-iso.linux-photon: Downloading linux-photon-4.0-develop-disk-0.vmdk...
==> vsphere-iso.linux-photon: Exporting linux-photon-4.0-develop-disk-0.vmdk...
==> vsphere-iso.linux-photon: Writing OVF descriptor linux-photon-4.0-develop.ovf...
==> vsphere-iso.linux-photon: Creating SHA1 manifest linux-photon-4.0-develop.mf...
==> vsphere-iso.linux-photon: Completed export to Open Virtualization Format (OVF).
    vsphere-iso.linux-photon: Closing sessions ....
Build 'vsphere-iso.linux-photon' finished after 7 minutes 28 seconds.
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