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

[github-actions[bot]]

This pull request changes the compat entry for the Turing package from 0.24, 0.25 to 0.24, 0.25, 0.26 for package ppl. 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.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1b7c135) 96.89% compared to head (d03aae6) 96.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #369 +/- ## ======================================= Coverage 96.89% 96.90% ======================================= Files 10 11 +1 Lines 387 388 +1 ======================================= + Hits 375 376 +1 Misses 12 12 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaGaussianProcesses/AbstractGPs.jl/pull/369/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses)

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