"cohttp-async" is constrained to an older version in the opam file

This library compiles with cohttp-async 6.3.0 with a very small change, so the opam file could allow this version (and doesn't need to restrict to `< 6.0.0`)

The change needed:
https://github.com/janestreet/cohttp_async_websocket/commit/f505da98ec79cb1d9be1eb984112d42f38c8b806

Start with the repository's opam file and inspect the linked commit f505da98ec79cb1d9be1eb984112d42f38c8b806 to identify the small compatibility change. Verify that the dependency constraint permits cohttp-async 6.3.0 and confirm the library compiles with that version.

Written by the indexing model from the issue text.