Update ctr requirement from 0.3 to 0.6

Updates the requirements on [ctr](https://github.com/RustCrypto/stream-ciphers) to permit the latest version.
- [Release notes](https://github.com/RustCrypto/stream-ciphers/releases)
- [Commits](RustCrypto/stream-ciphers@ctr-v0.3.0...ctr-v0.6.0)

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

Author: dependabot[bot]

protocols/secio/Cargo.toml

@@ -15,7 +15,7 @@ maintenance = { status = "deprecated" }
 [dependencies]
 aes-ctr = "0.3"
 aesni = { version = "0.6", features = ["nocheck"], optional = true }
-ctr = "0.3"
+ctr = "0.6"
 futures = "0.3.1"
 hmac = "0.9.0"
 lazy_static = "1.2.0"