Adapted Sherpa interface to support releases 2 and 3

Update CMake modules for Sherpa 3

Move CI to LCG_106a to pick up Sherpa 3.0.0

Modify CI to use Sherpa 2 for legacy builds

Implemented modification to use PHOTONS::AddRadiation function directly.

Removed unnecessary setting of blob status

Introduced finalise function inside EvtSherpaPhotons class to control the order of static destruction.

Updated setupEvtGen script for Sherpa 3

This is related with !39 and T250.

Run all FSR tests for the three supported Sherpa options and check that the results are the same as with the master branch irrespective of Sherpa version.