build(deps): bump bn.js

Bumps [bn.js](https://github.com/indutny/bn.js) from 4.12.1 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](https://github.com/indutny/bn.js/commits/v4.12.3)

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

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

Author: dependabot[bot]