remove vehicleModelDate in favor of modelDate

[VladimirAlexiev]

@trypuz @sopekmir @danbri
http://schema.org/vehicleModelDate and http://auto.schema.org/modelDate mean the same (have the same description), so one should be eliminated.
The specific property is in core while the generic property is in the "auto" extension, which is clearly a mistake.

#1650 and #1674 discuss this situation, but this pair seems to have fallen through the cracks. The auto extension documentation http://sdo-auto-fix.appspot.com/docs/automotive.html gives example only of modelDate but not vehicleModelDate (#1677 confirms that was the intention).

In the spirit of #1736, I propose to:

• kill vehicleModelDate
• move modelDate to core: http://schema.org/modelDate
• generalize the description to match the name: "The release date of a product model (often used to differentiate versions of the same make and model)" and the range to Product

[github-actions]

This issue is being tagged as Stale due to inactivity.