Update tokenizers requirement from 0.20 to 0.21

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] <support@github.com>

Author: dependabot[bot]

crates/bpe/benchmarks/Cargo.toml

@@ -23,4 +23,4 @@ bpe-openai = { path = "../../bpe-openai" }
 criterion = "0.5"
 rand = "0.8"
 tiktoken-rs = "0.6"
-tokenizers = { version = "0.20", features = ["http"] }
+tokenizers = { version = "0.21", features = ["http"] }