build(deps): bump formidable from 2.1.2 to 3.5.0 in /src

Bumps [formidable](https://github.com/node-formidable/formidable) from 2.1.2 to 3.5.0.
- [Release notes](https://github.com/node-formidable/formidable/releases)
- [Changelog](https://github.com/node-formidable/formidable/blob/master/CHANGELOG.md)
- [Commits](https://github.com/node-formidable/formidable/commits/v3.5.0)

---
updated-dependencies:
- dependency-name: formidable
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

src/package.json

@@ -43,7 +43,7 @@
     "express-session": "npm:@etherpad/express-session@^1.18.1",
     "fast-deep-equal": "^3.1.3",
     "find-root": "1.1.0",
-    "formidable": "^2.1.2",
+    "formidable": "^3.5.0",
     "http-errors": "^2.0.0",
     "js-cookie": "^3.0.5",
     "jsdom": "^20.0.0",