Restore backward compatibility with GitHub enterprise 2.20
Created by: eseliger
This PR restores compatibility with GitHub enterprise 2.20, which seems to be still used by customers. I think once we deprecate support for that version we should revert this PR as it's kind of a mess. We do so by fetching the instance version first and then constructing the GraphQL queries based on version constraints.
Any good ideas how to test this besides the manual tests I did? Not sure how much effort we want to put in this PR, given it's out-of-band support work for this sprint.
Closes #16043