XMP property `crs:CropUnits` is declared as `Integer` but it could be better `Closed choice of Integer`

Present since XMP Specification 2005, and still in the 2022 revision, `crs:CropUnits` is declared as an `Integer` but in the ends the specification enumerates all the possible values, so it should be better `Closed choice of Integer` as done for a many other properties, most of them in `exif` or `tiff` namespace, eg. `tiff:ResolutionUnit`. The issue is present both in the current specification [pdf document](https://github.com/adobe/XMP-Toolkit-SDK/blob/main/docs/XMPSpecificationPart2.pdf) or the [Markdown](https://github.com/adobe/xmp-docs/blob/master/XMPNamespaces/crs.md) rendition. Excerpt below:

This issue has been first created in the [pdf-issues](https://github.com/pdf-association/pdf-issues/issues/622) repository but moved to this one after the recommendation in the PDF TWG discussion.