Firefox 3.6 supported background-size: auto (#25356)

* Replace `background-size: auto` version ranges

As `auto` is the default value, it was very likely
supported as long as `background-size` was supported.

For Firefox, implementation happened before version 3, see:
- https://bugzilla.mozilla.org/show_bug.cgi?id=189519

* Update css/properties/background-size.json

css/properties/background-size.json

@@ -134,7 +134,7 @@
                 "version_added": "12"
               },
               "firefox": {
-                "version_added": "≤72"
+                "version_added": "3.6"
               },
               "firefox_android": "mirror",
               "ie": {