Switch to using OCaml 5.x

[alastairreid]

We have been using the LTS 4.14.x version for a while.
This avoided the risk of bugs in the 5.x releases and it is generally good to avoid using bleeding edge versions unless absolutely necessary.
But 4.14.x is now getting a bit old and it is due to stop being LTS soon (https://ocaml.org/tools/compiler-release-cycle).

So we should be able to move onto 5.x soon.
However, before doing this, we need to test it.
And, perhaps more importantly, we need to understand which (if any) of the 5.x releases is now the LTS release.