(run2) Rewrite CMake configuration in "modern CMake"

See LHCb!3126 (merged) LBCOMP-23, run2-patches version of !541 (merged)

Requires Analysis!837 (merged) Stripping!1677 (merged)

added Build legacy lhcb-new-cmake modernisation labels

• [2021-11-24 14:48] Validation started with lhcb-new-cmake#381
• [2021-11-25 00:29] Validation started with lhcb-new-cmake#382
• [2021-11-25 11:23] Validation started with lhcb-new-cmake#383
• [2021-11-26 00:32] Validation started with lhcb-new-cmake#384
• [2021-11-26 12:42] Validation started with lhcb-new-cmake#385
• [2021-11-27 00:39] Validation started with lhcb-new-cmake#386
• [2021-11-27 00:58] Validation started with lhcb-run2-gaudi-head#239
• [2021-11-27 00:58] Validation started with lhcb-run2-patches#1062
• [2021-11-27 00:59] Validation started with lhcb-run2-patches-dev4#888
• [2021-11-28 00:26] Validation started with lhcb-new-cmake#387
• [2021-11-28 00:37] Validation started with lhcb-run2-patches-dev4#889
• [2021-11-28 00:39] Validation started with lhcb-run2-gaudi-head#240
• [2021-11-28 00:40] Validation started with lhcb-run2-patches#1063
• [2021-11-29 00:30] Validation started with lhcb-run2-patches-dev4#890
• [2021-11-30 00:39] Validation started with lhcb-new-cmake#388
• [2021-11-30 00:51] Validation started with lhcb-run2-gaudi-head#241
• [2021-11-30 00:52] Validation started with lhcb-run2-patches-dev4#891
• [2021-11-30 00:58] Validation started with lhcb-run2-patches#1064

changed the description

added all-slots label

removed all-slots label

removed lhcb-new-cmake label

mentioned in commit 0cdaf7bf

merged