Fix to known platforms and clang 11 warning

follows up on !3062 (merged)

added Core all-slots modernisation labels

mentioned in merge request !3071 (merged)

/cc @clemenci

• [2021-05-26 00:04] Validation started with lhcb-g4-dev#850
• [2021-05-26 00:05] Validation started with lhcb-dd4hep#828
• [2021-05-26 00:06] Validation started with lhcb-gaudi-head#2937
• [2021-05-26 00:12] Validation started with lhcb-gaussino-fastsim#31
• [2021-05-26 00:13] Validation started with lhcb-gauss-lamarr#262
• [2021-05-26 00:15] Validation started with lhcb-gaussino#957
• [2021-05-26 00:18] Validation started with lhcb-head-2#250
• [2021-05-26 00:20] Validation started with lhcb-head#2953
• [2021-05-26 00:24] Validation started with lhcb-lcg-dev3#1642
• [2021-05-26 00:31] Validation started with lhcb-lcg-dev4#1657
• [2021-05-26 00:36] Validation started with lhcb-lcg-test#193
• [2021-05-26 01:16] Validation started with lhcb-sanitizers#865
• [2021-05-26 01:16] Validation started with lhcb-run3-cleanup#218
• [2021-05-26 01:25] Validation started with lhcb-dd4hep#828
• [2021-05-26 01:30] Validation started with lhcb-gaussino-fastsim#31
• [2021-05-26 01:32] Validation started with lhcb-head#2953

approved this merge request

mentioned in commit b214e4d6

merged

removed all-slots label