Mark http.headers.Content-Security-Policy.require-trusted-types-for as supported in Firefox preview only (#28449)

Author: ddbeck

http/headers/Content-Security-Policy.json

@@ -860,7 +860,7 @@
               "chrome_android": "mirror",
               "edge": "mirror",
               "firefox": {
-                "version_added": "145"
+                "version_added": "preview"
               },
               "firefox_android": "mirror",
               "oculus": "mirror",