LCG_87 Downgrade, 21.0-mc16a branch (2020.11.30.)
Switched the build of Athena (back) to LCG_87.
As discussed on ATLINFR-3850, using LCG_88 in this branch was quite a big mistake. So this is a first step needed in making the x86_64-centos7-gcc62-opt build of this branch functional.