Update base64 requirement from 0.20 to 0.21 (#51)

Updates the requirements on [base64](https://github.com/marshallpierce/rust-base64) to permit the latest version.
- [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/compare/v0.20.0...v0.21.0)

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

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Cargo.toml

@@ -38,7 +38,7 @@ panic = 'abort'
 aes-gcm = "0.10"
 anyhow = "1.0"
 argon2 = "0.4"
-base64 = "0.20"
+base64 = "0.21"
 bincode = "1.3"
 blake3 = "1.3"
 hex = "0.4"