Drop compatibility with LHCb v53r1

Two MR have been modified to preserve compatibility with LHCb v53r1 until all tests are looked at to move to LCG_101 and LHCb v53r5 (or higher).

Merging them makes all nightly slots, including a vanilla master for Gauss buildable.

Nevertheless the compatibility should be dropped and the relevant commits in the two MR should be reverted

• !766 (merged) commit bbe0c35c
• !791 (merged) commit af14611b