INSTALL: update the install instructions regarding librsvg.

Currently it's a mandatory option (and it has been the case for years,
ever since commit 43e218859b) so let's update the info.

Note that there are still discussions going on about this dependency and
it being hard or impossible to build on many platforms (which are stuck
on old C version, before the move to Rust). See #6821.
We'll see how it goes.

INSTALL.in

@@ -95,8 +95,8 @@ header files installed.
   8. For metadata access GIMP requires the gexiv2 @GEXIV2_REQUIRED_VERSION@ or newer library.
      It is hosted at: https://wiki.gnome.org/Projects/gexiv2
 
-  9. libpng, libjpeg, libtiff and lcms are hard dependencies that can
-     not be disabled.
+  9. libpng, libjpeg, libtiff, librsvg and lcms are hard dependencies
+     that can not be disabled.
 
  10. For MyPaint brushes, brushlib (libmypaint) @LIBMYPAINT_REQUIRED_VERSION@ is used.
      The libmypaint repository is hosted at:
@@ -132,7 +132,7 @@ header files installed.
  12. You may want to install other third party libraries or programs
      that are needed for some of the available plug-ins. We recommend
      to check that the following libraries are installed: openjpeg,
-     webkit, libmng, librsvg, libwmf, libaa and libgs (Ghostscript).
+     webkit, libmng, libwmf, libaa and libgs (Ghostscript).
 
  13. HEIF support depends on the libheif library. If you don't have
      access to pre-built packages, the code is available at:
@@ -318,9 +318,6 @@ These are:
      Help Browser plug-in, you can use --without-webkit to disable
      it explicitly.
 
-  --without-librsvg.  If for some reason you want to build GIMP without
-     SVG support, you can build --without-librsvg.
-
   --without-print.  If for some reason you don't want to build the Print
      plug-in based on the GtkPrint API, you can build with --without-print.