4.2.1 : bump Geant4 patch version for upstream biasing

The updated Geant4 patch enables biasing of volumes upstream of the target and the patch has no effect on any current samples which bias the target region or downstream volumes.

What's Changed

• bump Geant4 patch version by @tomeichlersmith in #93 and LDMX-Software/geant4#14

Full Changelog: 4.2.0...4.2.1