Upgrading guide should document use of a constraints file to avoid inadvertent Nautobot upgrade

[glennmatthews]

https://github.com/nautobot/cookiecutter-nautobot-app/blob/develop/nautobot-app/%7B%7B%20cookiecutter.project_slug%20%7D%7D/docs/admin/upgrade.md should probably have some boilerplate about using a constraints file with pip to avoid inadvertently updating to a new Nautobot version just because the latest release of the App requires a newer version. Probably something similar to how it's done in the Dockerfile, i.e.:

pip show nautobot | grep "^Version: " | sed -e 's/Version: /nautobot==/' > constraints.txt
pip install --constraint constraints.txt --upgrade {{ cookiecutter.plugin_name }}