all vocab links should be versioned

[TallTed]

Please note that my points are not philosophical. The text discusses the importance of using an un-changing vocab for VCs, and then uses a changing vocab for the example. The example would be much better (and more realistic) if it used versioned URIs for all schema.org terms, or switched to using an un-changing (i.e., already and more cleanly versioned) vocab.

Originally posted by @TallTed in https://github.com/w3c/vc-imp-guide/pull/54#issuecomment-525313912

We need to do a full review of this doc, and change most to versioned (especially, but probably not only, schema.org links).