fix(npf): fix newton under_relaxation in confined cells

[jdhughes-dev]

Modified gwf npf newton under_relaxation so that it is not applied in vertical columns where the lowest cell in a vertical column is confined.

Checklist of items for pull request

• Replaced section above with description of pull request
• Added new test or modified an existing test
• Ran ruff on new and modified python scripts in .doc, autotests, doc, distribution, pymake, and utils subdirectories.
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated definition files
• Updated develop.toml with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Removed checklist items not relevant to this pull request

For additional information see instructions for contributing and instructions for developing.