chore: fix 8.18 warnings #1566

Job	Run time
quick-build (supported, ubuntu-latest)	56s
opam-build (supported, ubuntu-latest)	2m 21s
quick-build (latest, ubuntu-latest)	1m 3s
nix	2m 14s
quick-build (dev, ubuntu-latest)	3m 31s
opam-build (8.17, ubuntu-latest)	2m 4s
build (supported)	1m 18s
build (latest)	1m 10s
opam-build (latest, ubuntu-latest)	1m 52s
opam-build (dev, ubuntu-latest)	6m 18s
build (dev, --warnings)	3m 37s
coqchk	0s
doc-alectryon	0s
doc-coqdoc	0s
doc-dep-graphs	0s
doc-timing	0s
install	0s
deploy-doc	0s
delete-artifacts	4s
	26m 28s

Provide feedback