Remove version of sihl in template

[gridbugs]

The goal of this change is to allow this project to be developed using dune package management, which wasn't possible before as the template project depended on a version of sihl other than the one in this repository. Admitedly it's not ideal that dune tries to include the template's dependencies in the dependency solution for this project, but a simple workaround is to remove the version constraint.

Setting the version of sihl in the template project to the version of sihl current in this project (4.0.0) would also work, but removing the constraint altogether feels like less maintenance.