Bump base64 from 0.4.2 to 0.9.3

Bumps [base64](https://github.com/marshallpierce/rust-base64) from 0.4.2 to 0.9.3.
- [Release notes](https://github.com/marshallpierce/rust-base64/releases)
- [Changelog](https://github.com/marshallpierce/rust-base64/blob/master/RELEASE-NOTES.md)
- [Commits](https://github.com/marshallpierce/rust-base64/commits/v0.9.3)

---
updated-dependencies:
- dependency-name: base64
  dependency-type: direct:production
...

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Author: dependabot[bot]