CompatHelper: bump compat for Gridap to 0.17, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Gridap package from 0.15.5 to 0.15.5, 0.17. 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.