bump to coq lang 0.8 #1532

Job	Run time
nix	3m 27s
quick-build (supported, ubuntu-latest)	3m 28s
quick-build (latest, ubuntu-latest)	4m 23s
quick-build (dev, ubuntu-latest)	3m 26s
opam-build (supported, ubuntu-latest)	6m 40s
opam-build (8.17, ubuntu-latest)	5m 26s
opam-build (latest, ubuntu-latest)	6m 32s
opam-build (dev, ubuntu-latest)	5m 18s
build (supported)	5m 22s
build (latest)	3m 54s
build (dev, --warnings)	4m 8s
doc-timing	1m 28s
doc-alectryon	18m 55s
doc-coqdoc	1m 26s
coqchk (supported)	2m 27s
coqchk (latest)	1m 56s
coqchk (dev)	1m 52s
doc-dep-graphs	8m 49s
install (supported)	1m 42s
install (latest)	1m 25s
install (dev)	1m 22s
deploy-doc	0s
delete-artifacts	8s
	1h 33m 34s

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