Bump elliptic and @ethersproject/signing-key

Bumps [elliptic](https://github.com/indutny/elliptic) and [@ethersproject/signing-key](https://github.com/ethers-io/ethers.js/tree/HEAD/packages/signing-key). These dependencies needed to be updated together.

Updates `elliptic` from 6.5.4 to 6.6.1
- [Commits](indutny/elliptic@v6.5.4...v6.6.1)

Updates `@ethersproject/signing-key` from 5.7.0 to 5.8.0
- [Release notes](https://github.com/ethers-io/ethers.js/releases)
- [Changelog](https://github.com/ethers-io/ethers.js/blob/v5.8.0/CHANGELOG.md)
- [Commits](https://github.com/ethers-io/ethers.js/commits/v5.8.0/packages/signing-key)

---
updated-dependencies:
- dependency-name: elliptic
  dependency-version: 6.6.1
  dependency-type: indirect
- dependency-name: "@ethersproject/signing-key"
  dependency-version: 5.8.0
  dependency-type: indirect
...

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Author: dependabot[bot]