CompatHelper: bump compat for Turing to 0.21 for package turing, (keep existing compat) (#403)

This pull request changes the compat entry for the `Turing` package from `0.18, 0.19, 0.20` to `0.18, 0.19, 0.20, 0.21` for package turing.
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.

Co-authored-by: David Widmann <[email protected]>

Author: github-actions[bot]

test/turing/Project.toml

@@ -6,5 +6,5 @@ Turing = "fce5fe82-541a-59a6-adf8-730c64b5f9a0"
 
 [compat]
 DynamicPPL = "0.19"
-Turing = "0.18, 0.19, 0.20"
+Turing = "0.21"
 julia = "1.3"