Add OCaml 5.3 to CI #965

Job	Run time
lint-opam	2m 45s
build-and-test (ubuntu-latest, 4.08, true)	3m 3s
build-and-test (ubuntu-latest, 4.08, false)	1m 56s
build-and-test (ubuntu-latest, 4.09, true)	2m 9s
build-and-test (ubuntu-latest, 4.09, false)	1m 57s
build-and-test (ubuntu-latest, 4.10, true)	2m 6s
build-and-test (ubuntu-latest, 4.10, false)	2m 2s
build-and-test (ubuntu-latest, 4.11, true)	2m 23s
build-and-test (ubuntu-latest, 4.11, false)	1m 58s
build-and-test (ubuntu-latest, 4.12, true)	2m 27s
build-and-test (ubuntu-latest, 4.12, false)	2m 3s
build-and-test (ubuntu-latest, 4.13, true)	3m 26s
build-and-test (ubuntu-latest, 4.13, false)	1m 58s
build-and-test (ubuntu-latest, 4.14, true)	2m 3s
build-and-test (ubuntu-latest, 4.14, false)	3m 1s
build-and-test (ubuntu-latest, 5.0, true)	2m 20s
build-and-test (ubuntu-latest, 5.0, false)	1m 59s
build-and-test (ubuntu-latest, 5.1, true)	2m 14s
build-and-test (ubuntu-latest, 5.1, false)	2m 7s
build-and-test (ubuntu-latest, 5.2, true)	2m 17s
build-and-test (ubuntu-latest, 5.2, false)	2m 4s
build-and-test (ubuntu-latest, 5.3, true)	2m 21s
build-and-test (ubuntu-latest, 5.3, false)	2m 9s
build-and-test (ubuntu-latest, ocaml-variants.5.3.0+options,ocaml-option-flambda,ocaml-option-mus...	2m 51s
build-and-test (macos-latest, 5.3, false)	3m 1s
build-and-test (windows-latest, 5.3, false)	5m 30s
	1h 4m 10s

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