Move to dune 1.7

[maiste]

Hi,
this PR moves the dune-project from 1.6.3 to 1.7.
For a future feature, dune needs to support dune lang 1.7 to allow to use virtual_modules stanza.

This PR changes the following things:

• dependency requirements:
  • dune must be >= dune 1.7
  • ipaddr is fixed to 2.8.0 to avoid dependency conflicts
  • lwt is moved to 4.0.0
• support js_of_ocaml 3.3 and tyxml 4.4.0 and update files to work with
• remove unused variable errors that stops the compilation with dune new version

[yurug]

Thank you @maiste !