Bump Dune lang version to 3.18 #29181

Job	Run time
Nix (macos-latest)	2m 20s
Nix (ubuntu-latest)	57s
Coverage (5.1.x)	9m 25s
Build (macos-latest, 5.1.x)	16m 13s
Build microbenchmarks	2m 34s
Build (ubuntu-latest, 5.1.x)	8m 50s
Coq 8.16.1	7m 27s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 40s
Documentation	1m 1s
Build (4.14.x, ubuntu-latest, true)	1m 41s
Create local opam switch (macos-latest, 5)	6m 46s
Build (4.14.x, macos-latest, true)	2m 40s
Create local opam switch (macos-latest, 4.14)	7m 18s
Build (5.1.x, macos-13, true)	3m 28s
Format	1m 9s
Create local opam switch (ubuntu-latest, 5)	5m 3s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 16s
Create local opam switch (ubuntu-latest, 4.14)	4m 57s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 3s
Build monorepo benchmark docker image	4m 4s
Build (4.08.x, ubuntu-latest, true)	1m 27s
Wasm_of_ocaml	10m 46s
Build (4.04.x, ubuntu-latest, true, true)	4m 45s
Build (4.02.x, ubuntu-latest, true)	3m 59s
	2h 10m 49s

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