Bump bitcoinjs-lib from 6.1.0 to 6.1.1 in /backend

Bumps [bitcoinjs-lib](https://github.com/bitcoinjs/bitcoinjs-lib) from 6.1.0 to 6.1.1.
- [Changelog](https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/CHANGELOG.md)
- [Commits](bitcoinjs/bitcoinjs-lib@v6.1.0...v6.1.1)

---
updated-dependencies:
- dependency-name: bitcoinjs-lib
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]