FF151 template element shadowrootassignment attribute (#29535)

Updated Firefox version_added from 'preview' to '151' and removed related flags.

Author: hamishwillee

html/elements/template.json

@@ -292,14 +292,7 @@
               "chrome_android": "mirror",
               "edge": "mirror",
               "firefox": {
-                "version_added": "preview",
-                "flags": [
-                  {
-                    "type": "preference",
-                    "name": "dom.shadowdom.shadowRootSlotAssignment.enabled",
-                    "value_to_set": "true"
-                  }
-                ]
+                "version_added": "151"
               },
               "firefox_android": "mirror",
               "oculus": "mirror",