Update base64 requirement from 0.13.0 to 0.20.0 (#326)

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/v0.13.0...v0.20.0)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Cargo.toml

@@ -27,7 +27,7 @@ rustls-tls-webpki-roots = ["__rustls-tls", "webpki-roots"]
 __rustls-tls = ["rustls", "webpki"]
 
 [dependencies]
-base64 = { version = "0.13.0", optional = true }
+base64 = { version = "0.20.0", optional = true }
 byteorder = "1.3.2"
 bytes = "1.0"
 http = { version = "0.2", optional = true }