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/web-browsers.scm

@@ -712,7 +712,7 @@ driven and does not detract you from your daily work.")
                   gsettings-desktop-schemas
                   cl-gobject-introspection
                   gtk+                  ; For the main loop
-                  webkitgtk             ; Required when we use its typelib
+                  webkitgtk-for-gtk3    ; Required when we use its typelib
                   gobject-introspection
                   pkg-config))
     (synopsis "Extensible web-browser in Common Lisp")