message: techdocs: restore event-source-polyfill pinning
diff: "@material-ui/lab": "4.0.0-alpha.57",
  "@material-ui/styles": "^4.10.0",
  "dompurify": "^2.2.9",
  - "event-source-polyfill": "^1.0.25",
  + "event-source-polyfill": "1.0.25",
  "git-url-parse": "^11.6.0",
  "jss": "~10.8.2",
  "lodash": "^4.17.21",
  