You'd need an 8th generation CPU or newer -- none of which will work on the system board you have.  Neither is there a system board you can use with that case which would take such a CPU. So --- if official Windows 11 support is your goal, a new system is the only way.  The Windows 11 CPU compatibility limitation can be circumvented, but if you choose to do that be advised that Microsoft has publicly stated it may ultimately start enforcing the 8th gen minimum CPU at some point -- when your system would cease to receive updates.    
Thank you.  
Precision Tower 3420, upgrade from i7-6700 for Windows 11?