build(deps): update base64 requirement from 0.13.0 to 0.20.0

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@v0.13.0...v0.20.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["letsencrypt", "acme"]
 categories = ["web-programming", "api-bindings"]
 
 [dependencies]
-base64 = "0.13.0"
+base64 = "0.20.0"
 hyper = { version = "0.14.18", features = ["client", "http1", "http2"] }
 hyper-rustls = { version = "0.23.0", default-features = false, features = ["http1", "http2", "native-tokio", "tls12"] }
 ring = { version = "0.16.20", features = ["std"] }