doc: change color contrast for accessibility

Change background color of Stability 1 banner. Previous color contrast
of 3.61:1 fails WCAG AA. New color contrast of 4.50:1 passes.

PR-URL: #35047
Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>

doc/api_assets/style.css

@@ -204,7 +204,7 @@ ol.version-picker li:last-child a {
 }
 
 .api_stability_1 {
-  background-color: #ec5315;
+  background-color: #ca5010;
 }
 
 .api_stability_2 {