refactor: use stdune in diagnostics #29499

Job	Run time
Nix (macos-latest)	2m 23s
Build (macos-latest, 5.1.x)	13m 4s
Nix (ubuntu-latest)	57s
Build (ubuntu-latest, 5.1.x)	8m 45s
Coq 8.16.1	7m 23s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 9s
Build (4.14.x, ubuntu-latest, true)	1m 30s
Documentation	58s
Build (4.14.x, macos-latest, true)	3m 2s
Format	1m 8s
Build (5.1.x, macos-13, true)	3m 43s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 58s
Wasm_of_ocaml	10m 43s
Create local opam switch (macos-latest, 5)	5m 12s
Create local opam switch (macos-latest, 4.14)	5m 10s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 18s
Build monorepo benchmark docker image	3m 39s
Build (4.08.x, ubuntu-latest, true)	1m 41s
Create local opam switch (ubuntu-latest, 5)	5m 0s
Build (4.04.x, ubuntu-latest, true, true)	4m 38s
Create local opam switch (ubuntu-latest, 4.14)	5m 15s
Build (4.02.x, ubuntu-latest, true)	3m 54s
Build microbenchmarks	2m 33s
	1h 55m 3s

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