Unless the update to the .NET Framework would affect consumers of the package or its API, I would increment the patch version number when updating dependencies.
Both the major and the minor version numbers are incremented with functional changes, with the major indicating backwards-incompatible changes and the minor indicating backwards-compatible changes. The patch version is for bug fixes, and I would argue other technical enhancements or enablements, that are backwards compatible.
The only case for updating something other than the patch version number would be if you introduced other functional changes at the same time as the dependency updates or if the dependency update could cause a breaking change for some consumers.
Partly, this depends on how your application is distributed. For example, if you are versioning an API that is consumed over HTTP, your dependencies have far less of an impact on consumers than if you are creating a library that gets included into a larger application, where upgrading technical dependencies could require consumers to install additional dependencies or invalidate existing installations upon update.