gnu: inkscape: Rename variable to inkscape/stable and hide package.

This will allow introducing a new 'inkscape' package that can be more freely
upgraded on the main branch.

* gnu/packages/inkscape.scm (inkscape): Rename to...
(inkscape/stable): ... this, and make it a hidden package.
* gnu/packages/admin.scm (wpa-supplicant-gui)[native-inputs]: Rename inkscape
input accordingly.
* gnu/packages/docbook.scm (dblatex): Likewise.
* gnu/packages/gnome-xyz.scm (arc-theme): Likewise.
* gnu/packages/python-xyz.scm (python-matplotlib-documentation): Likewise.
(python-nbconvert): Likewise.
* gnu/packages/xfce.scm (xfdesktop): Likewise.
* gnu/packages/xorg.scm (hackneyed-x11-cursors): Likewise.

gnu/packages/python-xyz.scm

@@ -6455,7 +6455,7 @@ toolkits.")
                          (string-append info "/matplotlib-figures"))))))))
     (native-inputs
      (list graphviz
-           inkscape
+           inkscape/stable
            python-colorspacious
            python-mpl-sphinx-theme
            python-scipy
@@ -13148,7 +13148,7 @@ time.")
                            "and not test_execute_widgets_from_nbconvert "
                            "and not test_execute_multiple_notebooks ")))))))))
     (inputs
-     (list inkscape pandoc))
+     (list inkscape/stable pandoc))
     (native-inputs
      (list python-ipykernel
            ;; Adding ipywidgets would create a cycle.