Bump big.js from 5.2.2 to 6.2.1

Bumps [big.js](https://github.com/MikeMcl/big.js) from 5.2.2 to 6.2.1.
- [Release notes](https://github.com/MikeMcl/big.js/releases)
- [Changelog](https://github.com/MikeMcl/big.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/MikeMcl/big.js/compare/v5.2.2...v6.2.1)

---
updated-dependencies:
- dependency-name: big.js
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

package.json

@@ -23,7 +23,7 @@
     "assert-never": "^1.2.1",
     "axios": "^1.1.2",
     "basic-auth": "^2.0.0",
-    "big.js": "^5.2.2",
+    "big.js": "^6.2.1",
     "bitcore-lib": "^8.25.8",
     "bluebird": "^3.7.2",
     "body-parser": "^1.16.0",