Replace WebP draft with newly published RFC

The WebP draft was published as RFC9649.

The `webp` shortname feels more appealing but we use `rfcxxxx` shortnames for
all other IETF RFCs, so moving `webp` to the list of former names.

specs.json

@@ -80,7 +80,6 @@
   },
   "https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-no-vary-search",
   "https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-rfc6265bis",
-  "https://datatracker.ietf.org/doc/html/draft-zern-webp/",
   "https://dom.spec.whatwg.org/",
   "https://drafts.css-houdini.org/css-typed-om-2/ delta",
   {
@@ -865,6 +864,12 @@
       "digest-headers"
     ]
   },
+  {
+    "url": "https://www.rfc-editor.org/rfc/rfc9649",
+    "formerNames": [
+      "webp"
+    ]
+  },
   "https://www.w3.org/2001/tag/doc/promises-guide",
   {
     "url": "https://www.w3.org/Consortium/Patent-Policy/",