Bug 1900458 - Ship @starting-style rule. r=layout-reviewers,webidl,saschanaz,emilio

Per https://groups.google.com/a/mozilla.org/g/dev-platform/c/bxSic3XP7oM,
we would like to ship @starting-style rule.

Differential Revision: https://phabricator.services.mozilla.com/D214946

dom/tests/mochitest/general/test_interfaces.js

@@ -330,7 +330,7 @@ let interfaceNamesInGlobalScope = [
   // IMPORTANT: Do not change this list without review from a DOM peer!
   { name: "CSSRuleList", insecureContext: true },
   // IMPORTANT: Do not change this list without review from a DOM peer!
-  { name: "CSSStartingStyleRule", insecureContext: true, nightly: true },
+  { name: "CSSStartingStyleRule", insecureContext: true },
   // IMPORTANT: Do not change this list without review from a DOM peer!
   { name: "CSSStyleDeclaration", insecureContext: true },
   // IMPORTANT: Do not change this list without review from a DOM peer!

modules/libpref/init/StaticPrefList.yaml

@@ -8662,7 +8662,7 @@
 # Whether @starting-style is enabled?
 - name: layout.css.starting-style-at-rules.enabled
   type: RelaxedAtomicBool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
   rust: true