Use a less restrictive ocaml/dune constraint

[anuragsoni]

With the removal of the the HTTP method handling we no longer use features that were specific to OCaml 4.06 (we were using Map.update before that). Ubuntu LTS still ships with OCaml 4.05 and there might be value in supporting versions at least as old as that. The code in the current condition should already compile with 4.05 so its just a matter of updating the version constraint in the opam file.

If a version older than 4.05 is needed, we'd need to replace the String.index_opt with something that works in older versions.

On a similar note we aren't using anything that truly needs a feature added only in dune 2. The autogenerated opam files are nice to have, but we don't really anticipate needing to touch the opam file too much so just maintaining that hand-written opam file can work. This would allow dropping the version constraint on dune.