Update rustls requirement from 0.20 to 0.21

Updates the requirements on [rustls](https://github.com/rustls/rustls) to permit the latest version.
- [Release notes](https://github.com/rustls/rustls/releases)
- [Changelog](https://github.com/rustls/rustls/blob/main/RELEASE_NOTES.md)
- [Commits](rustls/rustls@v/0.20.0...v/0.21.0)

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

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

Cargo.toml

@@ -53,7 +53,7 @@ log = "0.4"
 pin-project-lite = "0.2.8"
 num = { version = "0.4", optional = true }
 rand = { version = "0.8", optional = true }
-rustls = { version = "0.20", optional = true }
+rustls = { version = "0.21", optional = true }
 rustls-native-certs = { version = "0.6.0", optional = true }
 rustls-pemfile = { version = "1.0", optional = true }
 serde = "1.0"