irmin: mark as incompatible with Odoc.2.0.{1,2}
irmin: mark as incompatible with Odoc.2.0.{1,2}

These versions fail when attempting to build Irmin's documentation:

```
odoc: internal error, uncaught exception:
      Not_found
      Raised at Stdlib__map.Make.find in file "map.ml", line 137, characters 10-25
      Called from Odoc_xref2__Env.ElementsByName.remove in file "src/xref2/env.ml", line 116, characters 12-33
      Called from Odoc_xref2__Env.remove in file "src/xref2/env.ml", line 241, characters 11-52
      Called from Stdlib__list.fold_left in file "list.ml", line 121, characters 24-34
      Called from Odoc_xref2__Compile.include_.get_expansion in file "src/xref2/compile.ml", line 335, characters 20-63
      Called from Odoc_xref2__Compile.signature_items.(fun) in file "src/xref2/compile.ml", line 236, characters 21-35
      Called from Stdlib__list.fold_left in file "list.ml", line 121, characters 24-34
...
```

See https://github.com/ocaml/odoc/issues/793.

mirage/irmin,mirage/irmin