chore: fix 8.18 warnings #1569

Job	Run time
quick-build (supported, ubuntu-latest)	1m 8s
opam-build (supported, ubuntu-latest)	1m 52s
quick-build (latest, ubuntu-latest)	59s
nix	1m 3s
build (supported)	1m 18s
quick-build (dev, ubuntu-latest)	3m 57s
opam-build (8.17, ubuntu-latest)	2m 43s
build (latest)	1m 3s
opam-build (latest, ubuntu-latest)	2m 1s
build (dev, --warnings)	4m 38s
opam-build (dev, ubuntu-latest)	5m 7s
doc-coqdoc	0s
doc-dep-graphs	0s
doc-alectryon	0s
doc-timing	0s
coqchk	0s
install	0s
deploy-doc	0s
delete-artifacts	3s
	25m 52s

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