update proposal in conanfile to version 7.5.1

This updates PROPOSAL to its latest version.

Relevant for CORSIKA is the fix (#348), which fixes the energy calculation of Compton secondaries. This allows us to continue the merge of !451 (merged), where showers started to crash without this fix.

Furthermore, the LPM effect for photopair production has been added (#336), which allows us to continue the implementation of the LPM effect inside CORSIKA 8 which is currently done in !471 (closed).

See also the full release notes for 7.5.0, and the full release notes for 7.5.1