Switch to Guile-JSON 4.x.

* environment.scm <propagated-inputs>: Update to GUILE-JSON-4.
* jupyter/json.scm: Use (json parser) instead of (json).  Explain the
difference with what Guile-JSON provides.
* guix-jupyter-kernel.scm: Use (json parser) and (json builder) instead
of (json).
* guix/jupyter/kernel.scm: Likewise.
* guix/jupyter/proxy.scm: Likewise.
* jupyter/guile.scm: Likewise.
* jupyter/kernels.scm: Likewise.
* jupyter/messages.scm: Likewise.
* jupyter/servers.scm: Likewise.
* tests/guile.scm: Likewise.
* tests/kernels.scm: Likewise.
* tests/servers.scm: Likewise.
* configure.ac: Check for (json parser) rather than JSON.