build(deps): update async-openai requirement from 0.20.0 to 0.21.0

Updates the requirements on [async-openai](https://github.com/64bit/async-openai) to permit the latest version.
- [Release notes](https://github.com/64bit/async-openai/releases)
- [Commits](64bit/async-openai@async-openai-v0.20.0...async-openai-v0.21.0)

---
updated-dependencies:
- dependency-name: async-openai
  dependency-type: direct:production
...

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

Cargo.toml

@@ -20,5 +20,5 @@ trybuild = { version = "1.0", features = ["diff"] }
 syn = { version = "2.0", features = ["full", "extra-traits", "parsing"] }
 proc-macro2 = { version = "1.0", features = ["nightly"] }
 quote = "1.0"
-async-openai = "0.20.0"
+async-openai = "0.21.0"
 tokio = { version = "1.0", features = ["rt-multi-thread"] }