build(deps): bump sha2 from 0.9.3 to 0.9.4 (RustCrypto#175)

Bumps [sha2](https://github.com/RustCrypto/hashes) from 0.9.3 to 0.9.4.
- [Release notes](https://github.com/RustCrypto/hashes/releases)
- [Commits](RustCrypto/hashes@sha2-v0.9.3...sha2-v0.9.4)

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