Update etc/rgb.txt from X.Org upstream

Update etc/rgb.txt from X.Org upstream

The upstream version contains the following changes:

  2023-07-10  rgb: Make color entries uniform
  2014-07-06  Add aliases for colors that differ between X11 and CSS
  2014-07-06  Add missing colors from CSS Color Module Level 4
  2008-06-04  Nuke CVS version tags
  2003-11-14  R6.6 is the Xorg base-line

* etc/rgb.txt: Sync with the version in X.Org upstream
https://cgit.freedesktop.org/xorg/app/rgb/tree/rgb.txt
commit 0d2caecebf0e2a10994c22960921f366dd98d19a.  (Bug#66538)