Sherpa/SherpaOpenmpi: simplify for not using c++20 (apply for openmpi version)

To run the testing, add a comment with:

Bot test PACKAGE[ PACKAGE2 ...]|all in TOOLCHAIN|all [(only|also) for LABEL-COMPILER-BUILDTYPE] please!

To ensure patches required for existing LCG branches are not lost, please add the respective labels: alsoTargeting:LCG_102-patches, alsoTargeting:master, etc.