Added +detdesc platforms to LCG 102b_LHCB_Core and 102b

We agreed at the latest Core Software Meeting to switch, as of LCG 102, the default platforms to enable DD4hep and replace the special platforms +dd4hep with +detdesc equivalent ones for testing.

With this MR I'm adding the new platforms, while the default value of USE_DD4HEP has to be changed in LHCb, see lhcb/LHCb!3903 (merged), then the slots using LCG >= 102 need an updated list of platforms, see LHCbNightlyConf!934 (merged).

Commands:

./utils/new_lcg_version.py 102b_LHCB_Core \
    @LCG_102b_LHCB_Core/platforms.txt
for f in LCG_102b_LHCB_Core/x*.cmake ; do
    ln -sf .forward.cmake ${f/_LHCB_Core/} ;
done