CompatHelper: bump compat for DynamicPPL to 0.21 for package turing, (keep existing compat) (#430)

This pull request changes the compat entry for the `DynamicPPL` package from `0.20` to `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.

Author: github-actions[bot]

test/turing/Project.toml

@@ -5,6 +5,6 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 Turing = "fce5fe82-541a-59a6-adf8-730c64b5f9a0"
 
 [compat]
-DynamicPPL = "0.20"
+DynamicPPL = "0.20, 0.21"
 Turing = "0.21"
 julia = "1.6"