updated firefox support for multi values in color-mix()

Author: dletorey

css/types/color.json

@@ -251,14 +251,7 @@
                 "chrome_android": "mirror",
                 "edge": "mirror",
                 "firefox": {
-                  "version_added": "preview",
-                  "flags": [
-                    {
-                      "type": "preference",
-                      "name": "layout.css.color-mix-multi-color.enabled",
-                      "value_to_set": "true"
-                    }
-                  ]
+                  "version_added": "150"
                 },
                 "firefox_android": "mirror",
                 "oculus": "mirror",