Upgrade github-unix to yojson 3.0.0

[kit-ty-kate]

Requires ocaml/opam-repository#29055

[kit-ty-kate]

Actually only github-unix is impacted

[marijanp]

LGTM, checked out and build locally. I only received this error but this should be addressed in another PR

Alert ocaml_deprecated_auto_include: 
OCaml's lib directory layout changed in 5.0. The unix subdirectory has been
automatically added to the search path, but you should add -I +unix to the
command-line to silence this alert (e.g. by adding unix to the list of
libraries in your dune file, or adding use_unix to your _tags file for
ocamlbuild, or using -package unix for ocamlfind).