Bump bn.js in /ui

Bumps  and [bn.js](https://github.com/indutny/bn.js). These dependencies needed to be updated together.

Updates `bn.js` from 5.2.1 to 5.2.3
- [Release notes](https://github.com/indutny/bn.js/releases)
- [Changelog](https://github.com/indutny/bn.js/blob/master/CHANGELOG.md)
- [Commits](indutny/bn.js@v5.2.1...v5.2.3)

Updates `bn.js` from 4.12.0 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@v5.2.1...v5.2.3)

---
updated-dependencies:
- dependency-name: bn.js
  dependency-version: 5.2.3
  dependency-type: indirect
- dependency-name: bn.js
  dependency-version: 4.12.3
  dependency-type: indirect
...

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

Author: dependabot[bot]