chore: use 5.1.1 for tests #28109

Job	Run time
Nix (macos-latest)	2m 36s
Coverage (5.1.x)	10m 23s
Format	1m 1s
Nix (ubuntu-latest)	58s
Build (macos-latest, 5.1.x)	18m 34s
Build (ubuntu-latest, 5.1.x)	9m 15s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	5m 3s
Build microbenchmarks	1m 33s
Coq 8.16.1	7m 39s
Build (4.14.x, ubuntu-latest, true)	1m 29s
Create local opam switch (macos-latest, 5)	5m 20s
Build (4.14.x, macos-latest, true)	3m 15s
Create local opam switch (macos-latest, 4.14)	5m 41s
Documentation	1m 21s
Build (5.1.x, macos-13, true)	5m 33s
Create local opam switch (ubuntu-latest, 5)	4m 59s
Build monorepo benchmark docker image	3m 30s
Create local opam switch (ubuntu-latest, 4.14)	5m 14s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	16m 49s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	10m 22s
Build (4.08.x, ubuntu-latest, true)	1m 51s
Build (4.04.x, ubuntu-latest, true, true)	4m 49s
Build (4.02.x, ubuntu-latest, true)	4m 2s
	2h 11m 17s

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