Drop support for Coq 8.20 (#1853) #5050

Job	Run time
rocq-elpi	59s
rocq-core	58s
coq-elpi	57s
stdlib	56s
coq	1m 1s
hierarchy-builder	1m 15s
mathcomp-finmap	1m 41s
mathcomp-bigenough	1m 41s
mathcomp	2m 32s
mathcomp-analysis-single	2m 17s
mathcomp-classical	2m 17s
mathcomp-reals	2m 21s
mathcomp-experimental-reals	1m 15s
mathcomp-analysis	1m 15s
mathcomp-reals-stdlib	1m 19s
mathcomp-analysis-stdlib	1m 8s
	23m 52s

Provide feedback