Bump compat for Adapt to 4

[fhagemann]

Adapt v4.0.0 features breaking changes in Adapt.ndims, Adapt.eltype and Adapt.parent
(see https://github.com/JuliaGPU/Adapt.jl/releases/tag/v4.0.0)

DistributionsAD.jl seems to be using only Adapt.adapt here, so it should not be affected by the changes in Adapt v4.0.0.

Closes #262.

[avik-pal]

@devmotion can this be merged and tagged? It is holding back a downgrade CI

[devmotion]

I'll merge if tests pass.

It is holding back a downgrade CI

How is that possible? Doesn't a test failure of downgrade CI just indicate that the compat bounds of your package have to be fixed?

[avik-pal]

Had to drop compat with Adapt 3 at one place, and this is the only one currently in the dependency chain with no Adapt 4 compat. Should have been more clear -- it is a resolution failure not a test failure

[devmotion]

I still don't understand, how can you drop Adapt 3 if it is not supported yet by DistributionsAD? Shouldn't then the regular package tests, installation etc. fail as well?

[avik-pal]

The compat has not yet been dropped. I was running downgrade CI and after running through a few combinations this is only one which doesn't have Adapt 4, and then testing locally with this branch the dependency resolution works out