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.
This MR changes the default of USE_DD4HEP to true, while the new platforms are introduce in lcg-toolchains, see lhcb-core/lcg-toolchains!114 (merged).
I also switched specialized dd4hep reference files with detdesc ones.
In principle this should be merged when we switch the default LCG baseline to 102b or 103 (foreseen in January).