fix: update Cmdliner usage

[pm5]

Fix the following compilation errors:

 $ dune build
File "src/cli/slack_notify.ml", line 55, characters 2-20:
55 |   Cmdliner.Term.info "slack-notify" ~doc
       ^^^^^^^^^^^^^^^^^^
Error (alert deprecated): Cmdliner.Term.info
Use Cmd.info instead.
File "src/cli/slack_notify.ml", line 93, characters 4-8:
93 |     pure execute $ base_url $ token $ username $ channel $ icon_url $ icon_emoji
         ^^^^
Error (alert deprecated): Cmdliner.Term.pure
Use Term.const instead.
File "src/cli/slack_notify.ml", line 97, characters 8-26:
97 |   match Cmdliner.Term.eval (execute_t, info) with
             ^^^^^^^^^^^^^^^^^^
Error (alert deprecated): Cmdliner.Term.eval
Use Cmd.v and one of Cmd.eval* instead.

[Leonidas-from-XIV]

Thanks for your contribution!