update proposal version to 7.6.1

This updates the version of PROPOSAL to 7.6.0

This PR has the following consequences for CORSIKA 8:

• Fix incorrection low-energy ionization cross sections for electrons/positrons
• Include a better energy distribution sampling for triplet production (e -> 3e)
• Include a better sampling method for pairproduction production angles
• Use multiple scattering parametrization with improved performance (MoliereInterpol)

The full release notes are available on the release page