fix(package.json): Update dependency http-proxy-middleware to v0.21.0

Author: renovate-bot

package.json

@@ -43,7 +43,7 @@
     "file-loader": "1.1.11",
     "fs-extra": "6.0.1",
     "html-webpack-plugin": "4.3.0",
-    "http-proxy-middleware": "0.20.0",
+    "http-proxy-middleware": "0.21.0",
     "mini-css-extract-plugin": "0.4.5",
     "minimist": "1.2.5",
     "object-assign": "4.1.1",