chore: add x-maintenance-intent to dune opam files #29746

Job	Run time
Nix (macos-latest)	1m 51s
Format	1m 3s
Build microbenchmarks	2m 30s
Nix (ubuntu-latest)	52s
Build (macos-latest, 5.1.x)	14m 7s
Build (ubuntu-latest, 5.1.x)	8m 37s
Coq 8.16.1	7m 18s
Build (ocaml-variants.5.4.0+trunk, ubuntu-latest, true)	1m 30s
Documentation	53s
Create local opam switch (macos-latest, 5)	51s
Build (4.14.x, ubuntu-latest, true)	1m 33s
Create local opam switch (macos-latest, 4.14)	51s
Build monorepo benchmark docker image	2m 8s
Create local opam switch (ubuntu-latest, 5)	45s
Build (4.14.x, macos-latest, true)	2m 30s
Wasm_of_ocaml	10m 32s
Build (5.1.x, macos-13, true)	5m 7s
Create local opam switch (ubuntu-latest, 4.14)	50s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 31s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 21s
Build (4.08.x, ubuntu-latest, true)	1m 29s
Build (4.04.x, ubuntu-latest, true, true)	5m 42s
Build (4.02.x, ubuntu-latest, true)	3m 57s
	1h 37m 48s

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