build(deps): bump bn.js from 4.12.2 to 4.12.3 in /backend (#305)

Bumps [bn.js](https://github.com/indutny/bn.js) from 4.12.2 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/compare/v4.12.2...v4.12.3)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

backend/package-lock.json

@@ -2952,9 +2952,9 @@
       }
     },
     "node_modules/bn.js": {
-      "version": "4.12.2",
-      "resolved": "https://registry.npmjs.org/bn.js/-/bn.js-4.12.2.tgz",
-      "integrity": "sha512-n4DSx829VRTRByMRGdjQ9iqsN0Bh4OolPsFnaZBLcbi8iXcB+kJ9s7EnRt4wILZNV3kPLHkRVfOc/HvhC3ovDw==",
+      "version": "4.12.3",
+      "resolved": "https://registry.npmjs.org/bn.js/-/bn.js-4.12.3.tgz",
+      "integrity": "sha512-fGTi3gxV/23FTYdAoUtLYp6qySe2KE3teyZitipKNRuVYcBkoP/bB3guXN/XVKUe9mxCHXnc9C4ocyz8OmgN0g==",
       "license": "MIT"
     },
     "node_modules/brace-expansion": {