CompatHelper: bump compat for Interpolations to 0.15, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Interpolations package from 0.12, 0.13, 0.14 to 0.12, 0.13, 0.14, 0.15. 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.