Add header to ReactAllWarnings Meta-only file (#27819)

Adds missing headers to the generated file.

scripts/print-warnings/print-warnings.js

@@ -79,7 +79,19 @@ gs([
       const warningsArray = Array.from(warnings);
       warningsArray.sort();
       process.stdout.write(
-        `export default ${JSON.stringify(warningsArray, null, 2)};\n`
+        `/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @noformat
+ * @oncall react_core
+ */
+
+export default ${JSON.stringify(warningsArray, null, 2)};
+`
       );
     } else {
       process.stdout.write(