build(deps): update base64 requirement from 0.20 to 0.21

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](marshallpierce/rust-base64@v0.20.0...v0.21.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -18,7 +18,7 @@ maintenance = { status = "experimental" }
 
 [dependencies]
 backoff = "0.4"
-base64 = "0.20"
+base64 = "0.21"
 crossbeam-channel = "0.5"
 log = "0.4"
 rmp = "0.8"