Flag custom-state-pseudo-class as discontinued

The WICG spec moved to actual standardization with two parts:

1. The IDL part was moved to HTML in whatwg/html#8467
2. The CSS part is to be integrated in Selectors, see w3c/csswg-drafts#4805

This flags the spec as obsoleted by these two specs, using `selectors-4` for
the CSS part. The CSS WG resolution mentions Selectors 5 but it does not exist
yet.

specs.json

@@ -516,7 +516,14 @@
     }
   },
   "https://wicg.github.io/css-parser-api/",
-  "https://wicg.github.io/custom-state-pseudo-class/",
+  {
+    "url": "https://wicg.github.io/custom-state-pseudo-class/",
+    "standing": "discontinued",
+    "obsoletedBy": [
+      "html",
+      "selectors-4"
+    ]
+  },
   "https://wicg.github.io/datacue/",
   "https://wicg.github.io/deprecation-reporting/",
   "https://wicg.github.io/digital-goods/",