Add support for OCaml 4.14.3

[shonfeder]

Aims to close #72

We hit downstream impact of this in ocaml/opam-repository#29632

which gives errors like

File "dune-rules.inc", lines 65-72, characters 0-247:  
65 | (rule
66 |   (targets stdcompat_tests.ml)
67 |   (deps
68 |     (:gen tools/stdcompatpp.exe) stdcompat_tests.ml.in)
69 |     (action
70 |       (with-stdin-from stdcompat_tests.ml.in
71 |         (with-stdout-to stdcompat_tests.ml
72 |           (run %{gen} --source-type ocaml)))))
Fatal error: exception Not_found
Raised at Dune__exe__Compiler_version.make in file "tools/compiler_version.ml", line 144, characters 48-63
Called from Stdlib__Scanf.kscanf.apply in file "scanf.ml", line 1502, characters 27-32
Called from Dune__exe__Stdcompatpp.init_options in file "tools/stdcompatpp.mll", line 31, characters 21-65
Called from Dune__exe__Stdcompatpp.main in file "tools/stdcompatpp.mll", line 342, characters 16-31
Called from Dune__exe__Stdcompatpp in file "tools/stdcompatpp.mll", line 350, characters 8-15

I can't yet tell from the docs or older commits I've looked whether additional changes or code generation is needed for extending support for patch versions of the compiler. Figure I'll let CI or maintainers give me some pointers :)