gnu: Make webkitgtk-next the new webkitgtk.

As of version 2.42.0, the WebKit 6.0 typelib is considered stable.
With GNOME itself moving to GTK4 as time marches on, we should reflect
the fact that this is the preferred Webkit for future applications.

* gnu/packages/webkit.scm (webkitgtk-next): Remove variable.  Inline logic…
(webkitgtk): … here.  Move old value …
(webkitgtk-for-gtk3): … here.
(webkitgtk-with-libsoup2): Inherit from webkitgtk-for-gtk3.
* gnu/packages/geo.scm (gnome-maps): Adjust accordingly.
* gnu/packages/gnome.scm (gnome-initial-setup, sushi)
(cambalache, devhelp, devhelp-with-libsoup2, gnome-online-accounts)
(gnome-online-accounts-3.44, evolution-data-server, evolution-data-server-3.44)
(lollypop, evolution, gnome-boxes, geary, komikku): Likewise.
* gnu/packages/guile-xyz.scm (guile-gi, nomad): Likewise.
* gnu/packages/lisp-xyz.scm (sbcl-cl-webkit): Likewise.
* gnu/packages/mail.scm (balsa): Likewise.
* gnu/packages/mate.scm (atril): Likewise.
* gnu/packages/music.scm (ctrlr): Likewise.
* gnu/packages/rednotebook.scm (rednotebook): Likewise.
* gnu/packages/syndication.scm (gfeeds): Likewise.
* gnu/packages/telegram.scm (telegram-desktop): Likewise.
* gnu/packages/vnc.scm (remmina): Likewise.
* gnu/packages/web-browsers.scm (nyxt): Likewise.

gnu/packages/lisp-xyz.scm

@@ -5065,7 +5065,7 @@ is a library for creating graphical user interfaces.")
     (inputs
      `(("cffi" ,sbcl-cffi)
        ("cl-cffi-gtk" ,sbcl-cl-cffi-gtk)
-       ("webkitgtk" ,webkitgtk)))
+       ("webkitgtk" ,webkitgtk-for-gtk3)))
     (native-inputs
      `(;; Tests seem to need Xorg.
        ;; ("xorg-server" ,xorg-server-for-tests)