CompatHelper: bump compat for "SpecialFunctions" to "2"

[github-actions]

This pull request changes the compat entry for the SpecialFunctions package from 1.6 to 1.6, 2.

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.

[DhairyaLGandhi]

https://github.com/FluxML/Zygote.jl/runs/4302272011?check_suite_focus=true#step:6:1077 @ChrisRackauckas is there a known breakage with sde layers in the DiffEqFlux tests? Seems like some gradients are expected to be non zero aren't being populated

[ChrisRackauckas]

That's not a Zygote issue. I need to dig into that one.

[DhairyaLGandhi]

@carlo, PPL tests break with this too, I think we should ask @devmotion if those are known to break as of now.

[devmotion]

Why was this merged without testing it? There's no rush here, the new version does not contain any new features but only fixes a very old type piracy of factorial. Many packages are not compatible with it yet, eg ForwardDiff and hence also Distributions do not support it yet, and hence it can't be tested here.

[devmotion]

In any case, thanks for the ping @DhairyaLGandhi! The DynamicPPL test errors are known and caused by Distributions. The latest release of PDMats exposed some bugs, they are fixed in the latest release of Distributions.

[CarloLucibello]

Sorry for that, I didn't notice that SpecialFunctions was still blocked by upstream packages in test. Can revert if needed, although I think that is not necessary since the release has only very specific breakings