Solved my own question.  Displayport is backwards compatible. A Displayport 2.1 or 2.0 cable will work fine with my monitor.
I will soon receive my XPS 8960. It has a graphics card that supports Displayport 2.1.  My beloved but elderly monitor has a Displayport 1.1 connector. Is DP 2.1 backwards compatible with DP 1.1?  If the answer is (hopefully) yes, can I use a DP 2.1 cable to connect my monitor? Thanks for your help!
DisplayPort Backwards Compatibility