Drop CI testing of 8.16 (#1708) #688

Job	Run time
install (dev, 4.11.1, ubuntu-latest)	1h 12m 10s
install (dev, 4.11.1, macos-latest)	58m 48s
install (dev, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opam-r...	2h 48m 22s
install (8.18.0, 4.11.1, ubuntu-latest)	40m 42s
install (8.18.0, 4.11.1, macos-latest)	1h 5m 33s
install (8.18.0, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opa...	1h 37m 48s
install (8.17.0, 4.11.1, ubuntu-latest)	1h 2m 2s
install (8.17.0, 4.11.1, macos-latest)	59m 27s
install (8.17.0, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opa...	1h 36m 27s
	12h 1m 19s

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