CompatHelper: bump compat for Soss to 0.21 for package docs, (keep existing compat)

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This pull request changes the compat entry for the Soss package from 0.20 to 0.20, 0.21 for package docs. 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.

[sethaxen]

Currently blocked by https://github.com/cscherrer/SampleChainsDynamicHMC.jl/issues/20. Also, the new Ubuntu LTS appears to cause issues with PyCall, which is breaking our CI.

[sethaxen]

Now blocked by https://github.com/cscherrer/Soss.jl/issues/337