Actually update the newly created port lang/mono5.20 to 5.20.

This port is a WIP. It compiles and packages on both amd64 and powerpc64.
However, the ports depending on it are still under test. As such, it is
intentionally not yet connected to the build. I will continue to work
on this in the coming days.

This corresponds to the most recent patches in 238089 for 5.x.

Note: this port depends on the obsolete python2.7 (it is known that
it will not work with python3.X). Testing vs. tauthon is TBA.

PR: 238089
Submitted by: cem