Fix build with latest Vala 0.56.x

[bobby285271]

Previously #637. With vala@bac7c570 the failure is now on 0.56.x as well.

../lib/Widgets/Utils.vala:269.38-269.45: error: Argument 1: Cannot convert from `unowned uint8[]' to `unowned string'
  269 |         css_provider.load_from_data (css.data);
      |                                      ^~~~~~~~  
../lib/Widgets/Utils.vala:269.9-269.46: error: 1 missing arguments for `void Gtk.CssProvider.load_from_data (string, ssize_t)'
  269 |         css_provider.load_from_data (css.data);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

(CI failure does not seem to be related?)

[tintou]

Let's wait for Vala upstream to clarify on this breakage https://gitlab.gnome.org/GNOME/vala/-/issues/1478

[tintou]

This will be fixed in next Vala 0.56.x version (see https://gitlab.gnome.org/GNOME/vala/-/commit/8895fdbf2fa1ece975110e6555cbb36692dc4096 )

[bobby285271]

Looks like the fix is applied to 0.58 as well: https://gitlab.gnome.org/GNOME/vala/-/issues/1478#note_1827790 so we probably want to revert #637?