build(deps): update tokenizers requirement from 0.20.0 to 0.21.0

Updates the requirements on [tokenizers](https://github.com/huggingface/tokenizers) to permit the latest version.
- [Release notes](https://github.com/huggingface/tokenizers/releases)
- [Changelog](https://github.com/huggingface/tokenizers/blob/main/RELEASE.md)
- [Commits](huggingface/tokenizers@v0.20.0...v0.21.0)

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

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

Cargo.toml

@@ -20,7 +20,7 @@ reqwest = { version = "0.12.3", features = ["json", "stream"] }
 serde = { version = "1.0.197", features = ["derive"] }
 serde_json = "1.0.115"
 thiserror = "2.0.0"
-tokenizers = { version = "0.20.0", default-features = false, features = ["onig", "esaxx_fast"] }
+tokenizers = { version = "0.21.0", default-features = false, features = ["onig", "esaxx_fast"] }
 
 [dev-dependencies]
 dotenv = "0.15.0"