Drop support for Coq 8.20 #5048

Job	Run time
rocq-core	47s
rocq-elpi	1m 30s
coq	49s
coq-elpi	58s
stdlib	52s
mathcomp-finmap	1m 6s
mathcomp-bigenough	57s
hierarchy-builder	56s
mathcomp-analysis-single	26m 50s
mathcomp	11m 45s
mathcomp-classical	7m 13s
mathcomp-reals	2m 6s
mathcomp-experimental-reals	1m 51s
mathcomp-reals-stdlib	1m 31s
mathcomp-analysis	15m 5s
mathcomp-analysis-stdlib	1m 37s
	1h 15m 53s

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