doc: change stablility-2 color for accessibility

Current contrast ratio (2.51) does not pass WCAG AA. This PR increases
the contrast to 4.50, which passes WCAG AA. The color, of course, is
still green.

PR-URL: #35061
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>

doc/api_assets/style.css

@@ -207,7 +207,7 @@ ol.version-picker li:last-child a {
 }
 
 .api_stability_2 {
-  background-color: #4eba0f;
+  background-color: #5a8147;
 }
 
 .api_metadata {