fix: replace gradient background with solid color for nq-card component

Author: onmax

packages/nimiq-css/src/css/utilities.css

@@ -631,9 +631,7 @@
 
 .nq-card {
   & {
-    --nq-gradient-from: var(--colors-neutral-gradient-from);
-    --nq-gradient-to: var(--colors-neutral-gradient-to);
-    background-image: radial-gradient(at 100% 100% in oklab, var(--nq-gradient-from) 0%, var(--nq-gradient-to) 100%);
+    background-color: var(--colors-neutral-50);
     color: var(--colors-neutral);
     border-radius: 0.5rem;
     display: flex;