Drop support for Coq 8.20 (#1853) #1473

Job	Run time
rocq-core	55s
coq	50s
mathcomp-analysis-single	1m 53s
mathcomp-classical	1m 9s
mathcomp-reals	2m 34s
mathcomp-analysis	2m 30s
mathcomp-experimental-reals	32s
mathcomp-reals-stdlib	32s
ssprove	-1s
mathcomp-analysis-stdlib	-1s
mathcomp-infotheo	0s
	10m 53s

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