Bump bn.js from 4.11.9 to 4.12.3 in /flowauth/frontend

Bumps [bn.js](https://github.com/indutny/bn.js) from 4.11.9 to 4.12.3.
- [Release notes](https://github.com/indutny/bn.js/releases)
- [Changelog](https://github.com/indutny/bn.js/blob/master/CHANGELOG.md)
- [Commits](indutny/bn.js@v4.11.9...v4.12.3)

---
updated-dependencies:
- dependency-name: bn.js
  dependency-version: 4.12.3
  dependency-type: indirect
...

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Author: dependabot[bot]