ceratopsian: Remove the "13" from login background and Plymouth theme

Now that Debian 13 has been released, the version of desktop-base in
testing and unstable should stop identifying itself as Debian 13
(it's more like an early prerelease of Debian 14).

None of the other assets seem to include a version number.

Signed-off-by: Simon McVittie <smcv@debian.org>

ceratopsian-theme/login/background.svg

@@ -296,18 +296,6 @@
          id="path3084"
          style="fill:#85a0b4;fill-opacity:1" />
     </g>
-    <g
-       id="g3258"
-       transform="matrix(0.89684088,0,0,0.85199883,109.15354,152.43201)">
-      <path
-         d="m 1008.9107,1051.1401 q 0,0.8305 0,1.538 0,0.6767 0,1.3534 0.031,0.6768 0.031,1.415 0.031,0.7075 0.062,1.5688 0.031,0.7382 0.062,1.2304 0.061,0.4921 0.1231,0.8305 0.092,0.3384 0.1845,0.5845 0.092,0.2153 0.1846,0.4306 -0.6152,-0.062 -1.415,-0.092 -0.769,-0.062 -1.5688,-0.062 -0.6152,0 -1.1688,0.031 -0.5537,0.031 -1.2612,0.123 0.092,-0.3076 0.1538,-0.5844 0.092,-0.2769 0.1538,-0.6767 0.062,-0.4307 0.092,-1.0767 0.031,-0.6459 0.061,-1.7225 l 0.062,-4.3065 v -4.614 q 0,-2.0302 -0.031,-3.2606 -0.031,-1.2304 -0.092,-1.4765 -0.9843,0.3384 -1.8764,0.7075 -0.892,0.3691 -1.7225,0.892 l 0.062,-1.0151 q 0,-0.6459 -0.062,-1.0766 -0.031,-0.4614 -0.092,-0.7997 0.523,-0.1231 1.5073,-0.3692 0.9843,-0.246 2.1532,-0.6152 1.1997,-0.3998 2.3993,-0.9228 1.2304,-0.5229 2.2148,-1.1689 -0.062,0.7383 -0.1231,1.3227 -0.031,0.5537 -0.062,1.415 0,0.8613 -0.031,2.307 0,1.4458 0,3.9373 z"
-         style="font-weight:bold;font-size:24.8469px;font-family:Poppl-Laudatio;-inkscape-font-specification:'Poppl-Laudatio, Bold';white-space:pre;fill:#85a0b4;stroke:none;stroke-width:9.01383"
-         id="path3249" />
-      <path
-         d="m 1016.2305,1038.3745 q 0.8921,0 1.6611,0.092 0.7997,0.092 1.5072,0.3383 0.3384,0.1231 0.8613,0.4307 0.5537,0.2768 1.0766,0.8613 0.5229,0.5536 0.8921,1.4149 0.3999,0.8613 0.3999,2.1533 0,0.769 -0.2154,1.5687 -0.1845,0.7998 -0.6459,1.5073 -0.4614,0.6767 -1.1997,1.1996 -0.7382,0.523 -1.7841,0.7383 l -0.6767,0.123 0.7075,0.092 q 2.3993,0.3998 3.5374,1.661 1.1689,1.2612 1.1689,3.3837 0,1.6303 -0.6152,2.8299 -0.5845,1.1689 -1.3842,1.8764 -0.8921,0.7998 -2.3686,1.2304 -1.4457,0.4306 -3.3221,0.4306 -0.1845,0 -0.6767,-0.031 -0.4922,0 -1.1689,-0.061 -0.6767,-0.062 -1.4457,-0.1846 -0.7383,-0.123 -1.4765,-0.3691 0.123,-0.3999 0.2153,-0.769 0.1231,-0.3691 0.1846,-0.769 0.092,-0.4306 0.123,-0.9228 0.062,-0.4922 0.1231,-1.1689 0.769,0.6152 1.6918,0.8613 0.9536,0.2461 1.9379,0.2461 0.9536,0 1.6303,-0.2769 0.6767,-0.2768 1.1074,-0.7075 0.4306,-0.4614 0.6152,-1.015 0.1845,-0.5845 0.1845,-1.1689 0,-1.0151 -0.3998,-1.6303 -0.3999,-0.646 -1.0459,-1.0151 -0.6152,-0.3691 -1.415,-0.4922 -0.7997,-0.123 -1.6303,-0.123 -0.7997,0 -1.1689,0.062 -0.3691,0.031 -0.7997,0.092 0.031,-0.3999 0.031,-0.769 0.031,-0.3691 0.031,-0.769 -0.031,-0.4306 -0.031,-0.7998 0,-0.3998 -0.031,-0.8305 0.2461,0.031 0.4921,0.062 0.2461,0 0.646,0 0.3999,0 1.1689,-0.031 0.769,-0.062 1.538,-0.3691 0.769,-0.3076 1.3227,-0.9536 0.5844,-0.6459 0.5844,-1.8456 0,-1.2304 -0.8305,-2.0609 -0.7997,-0.8613 -2.5223,-0.8613 -1.3227,0 -1.9994,0.3076 -0.6768,0.3076 -1.0459,0.5537 0.1846,-1.415 0.2153,-2.184 0.031,-0.769 0,-1.4458 0.8306,-0.1845 1.7841,-0.3383 0.9844,-0.1538 2.4609,-0.1538 z"
-         style="font-weight:bold;font-size:24.8469px;font-family:Poppl-Laudatio;-inkscape-font-specification:'Poppl-Laudatio, Bold';white-space:pre;fill:#85a0b4;stroke:none;stroke-width:9.01383"
-         id="path3251" />
-    </g>
     <g
        id="g3070-3"
        transform="matrix(0.27064872,0,0,0.27064872,903.88697,1033.6588)"