user build under Windows: provide libcerf2

https://bornagainproject.org/20.0/installation/building/windows/ refers to libcerf1.14.win64.zip from https://computing.mlz-garching.de/download/WinLibs. Don't we currently depend on libcerf2?

• Provide Win zip for libcerf2.3 (libcerf#11 (closed))
• Change https://bornagainproject.org/20.0/installation/building/windows: new download location for libcerf win zip package is https://jugit.fz-juelich.de/mlz/libcerf/-/releases, not https://computing.mlz-garching.de/download/WinLibs
• Remove libcerf from https://computing.mlz-garching.de/download/WinLibs