Cap tokio-util below 0.7.20 in Rust SDK template to keep 1.83 MSRV #2002

Tracks PR #2001.

Plan

Goal: Keep generated Rust SDKs building and publishing on rustc 1.83 after tokio-util 0.7.20 (which needs rustc 1.85) broke the sdk-for-rust 0.16.0 publish.

What changes:

• templates/rust/Cargo.toml.twig: add tokio-util = ">=0.7, <0.7.20" next to the existing hyper-util cap (PR fix(rust): cap tokio-util below 0.7.20 to keep the 1.83 MSRV #2001).
• The same cap is applied in the SDK repo by fix: cap tokio-util below 0.7.20 to keep the 1.83 MSRV sdk-for-rust#32.

Remaining steps:

1. Merge fix: cap tokio-util below 0.7.20 to keep the 1.83 MSRV sdk-for-rust#32 and re-run its publish workflow from main to publish 0.16.0.

Done when:

• fix(rust): cap tokio-util below 0.7.20 to keep the 1.83 MSRV #2001 is merged
• fix: cap tokio-util below 0.7.20 to keep the 1.83 MSRV sdk-for-rust#32 is merged and 0.16.0 is published