CompatHelper: bump compat for "Adapt" to "2.0"

[github-actions]

This pull request changes the compat entry for the Adapt package from 1 to 1, 2.0.

This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[andreasnoack]

@piever Do you know why the tests are failing on older version of Julia?

[piever]

It was a bit of a riddle. As far as I understand, the way to get interoperability with GPU things is to overload Adapt, but to actually use it you need to also import GPUArrays, which (using type piracy) makes sure that Adapt.adapt(Array, x) does the right thing. I think newer Adapt needs a newer GPUArrays, which requires julia 1.3, so tests fail on older julia (it ends up installing a very old GPUArrays version, probably from before there were compat bounds at all).

I can probably test on a custom type (rather than Array) so that I can drop the GPUArrays test dependency (#133).

I've opened an issue on GPUArrays for the type piracy: JuliaGPU/GPUArrays.jl#296