Describe context as API changes instead of as federation

[michielbdejong]

The reason we are doing this is as described in https://github.com/SURFnet/surf-token-based-access/blob/main/phase-2/osw-abstract.md.
But others may understand its value better if instead of loose federation we mention evolving APIs. That's how I wrote it down now in the abstract of #73 (comment)

[michielbdejong]

We discussed this today, and I think we didn't explicitly decided for or against this, but instead agreed to iterate over the document in general more in the coming week.