CompatHelper: bump compat for Turing to 0.26, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Turing package from 0.20, 0.21, 0.22, 0.23, 0.24, 0.25 to 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26.
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.

[storopoli]

@torfjelde of @devmotion, are there any changes in how logpdf is exported in Turing.jl 0.26?
I went to the changelog for Turing and DynamicPPL new versions but I could not find anything. I don't know why the tests are failing.

[yebai]

This should be fixed by TuringLang/DistributionsAD.jl#256

[storopoli]

Now we need a new release on TuringLang/DistributionsAD.jl and a new release on TuringLang/Turing.jl

[yebai]

We only need a new DistributionsAD release, which is done. I triggered a re-run of CI; this PR should be ready for merging once CI passes.