docs(util-base64): add deprecation in README of runtime specific modules (#4173)

Author: trivikr

packages/util-base64-browser/README.md

@@ -2,3 +2,7 @@
 
 [![NPM version](https://img.shields.io/npm/v/@aws-sdk/util-base64-browser/latest.svg)](https://www.npmjs.com/package/@aws-sdk/util-base64-browser)
 [![NPM downloads](https://img.shields.io/npm/dm/@aws-sdk/util-base64-browser.svg)](https://www.npmjs.com/package/@aws-sdk/util-base64-browser)
+
+> Deprecated package
+
+This internal package is deprecated in favor of [@aws-sdk/util-base64](https://www.npmjs.com/package/@aws-sdk/util-base64).

packages/util-base64-node/README.md

@@ -2,3 +2,7 @@
 
 [![NPM version](https://img.shields.io/npm/v/@aws-sdk/util-base64-node/latest.svg)](https://www.npmjs.com/package/@aws-sdk/util-base64-node)
 [![NPM downloads](https://img.shields.io/npm/dm/@aws-sdk/util-base64-node.svg)](https://www.npmjs.com/package/@aws-sdk/util-base64-node)
+
+> Deprecated package
+
+This internal package is deprecated in favor of [@aws-sdk/util-base64](https://www.npmjs.com/package/@aws-sdk/util-base64).