[ci] Bump bignumber.js from 9.1.1 to 9.3.1 in /load-tester (#4018)

Bumps [bignumber.js](https://github.com/MikeMcl/bignumber.js) from 9.1.1 to 9.3.1.
- [Release notes](https://github.com/MikeMcl/bignumber.js/releases)
- [Changelog](https://github.com/MikeMcl/bignumber.js/blob/main/CHANGELOG.md)
- [Commits](MikeMcl/bignumber.js@v9.1.1...v9.3.1)

---
updated-dependencies:
- dependency-name: bignumber.js
  dependency-version: 9.3.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]