Change targetLH2 material from LiquidHydrogen back to G4_lH2

[zdemirog]

This PR reverts target material from LiquidHydrogen back to G4_lH2.

• The target material was changed from G4_lH2 to LiquidHydrogen in updates on LH2 target properties: implementing realistic temperature, pressure, density values for LH2 #630
• While testing latest develop branch, I saw significant decrease in quartz tile rates for physics generators. This issue wasn't present in earlier versions of develop branch (after PR623/624). I ran a quick comparison using 50k Moller events with identical random seeds, and the results show that LiquidHydrogen leads lower rate values.
• More simulations are needed to understand why switching to LiquidHydrogen caused this rate drop. Due to low slurm priority, I can't run full tests right now. So, reverting to G4_lH2 is the safer temporary solution until we can debug the issue.