Update base64 requirement from 0.21.0 to 0.22.0 (#458)

Updates the requirements on [base64](https://github.com/marshallpierce/rust-base64) to permit the latest version.
- [Changelog](https://github.com/marshallpierce/rust-base64/blob/master/RELEASE-NOTES.md)
- [Commits](marshallpierce/rust-base64@v0.21.0...v0.22.0)

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

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

Author: dependabot[bot]

metrics-exporter-prometheus/Cargo.toml

@@ -30,7 +30,7 @@ metrics-util = { version = "^0.16", path = "../metrics-util", default-features =
 thiserror = { version = "1", default-features = false }
 quanta = { version = "0.12", default-features = false }
 indexmap = { version = "2.1", default-features = false, features = ["std"] }
-base64 = { version = "0.21.0", default-features = false, features = ["std"] }
+base64 = { version = "0.22.0", default-features = false, features = ["std"] }
 
 # Optional
 hyper = { version = "1.1", features = [ "server", "client" ],  optional = true }