update to ocaml 5.1

[kmarker1101]

Looks like the test-runner uses ocaml 5.1. Not sure how many people use the dev container but was thinking we should update to 5.1 to match the test-runner. I also updated the README.

[kmarker1101]

Looks like word-count fails due to a depreciated module. Should be able to fix but not sure if we should or not.

File "word_count.ml", line 28, characters 11-26:
28 |       then Caml.String.sub w 1 (len - 2)
                ^^^^^^^^^^^^^^^
Error: Unbound module Caml.String

Can we use just String.sub here?

[georgyo]

I have those fixes in #500, but I cannot self approve it.

[BNAndras]

#500 is merged in now so you can try rebasing.