build(deps): update aws-smithy-runtime requirement from <=1.2 to <=1.6

Updates the requirements on [aws-smithy-runtime](https://github.com/smithy-lang/smithy-rs) to permit the latest version.
- [Release notes](https://github.com/smithy-lang/smithy-rs/releases)
- [Changelog](https://github.com/smithy-lang/smithy-rs/blob/main/CHANGELOG.md)
- [Commits](https://github.com/smithy-lang/smithy-rs/commits)

---
updated-dependencies:
- dependency-name: aws-smithy-runtime
  dependency-type: direct:production
...

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

Cargo.toml

@@ -27,7 +27,7 @@ features = ["derive"]
 [dev-dependencies]
 hex = "0.4"
 aws-config = { version = "<=1.1" }
-aws-smithy-runtime = { version = "<=1.2" }
+aws-smithy-runtime = { version = "<=1.6" }
 
 [features]
 default = ["key_openssl_pkey"]