fix: update hex color format output to use rgb() notation

Author: SteKoe

spring-boot-admin-server-ui/src/main/java/de/codecentric/boot/admin/server/ui/config/CssColorUtils.java

@@ -29,7 +29,7 @@ public static String hexToRgb(String color) {
 		if (!HEX_RGB_PATTERN.matcher(color).matches()) {
 			throw new IllegalArgumentException(String.format("Invalid hex rgb format %s", color));
 		}
-		return String.format("%s, %s, %s", Integer.valueOf(color.substring(1, 3), 16),
+		return String.format("rgb(%s, %s, %s)", Integer.valueOf(color.substring(1, 3), 16),
 				Integer.valueOf(color.substring(3, 5), 16), Integer.valueOf(color.substring(5, 7), 16));
 	}