Update testcontainers requirement from 0.20 to 0.21

Updates the requirements on [testcontainers](https://github.com/testcontainers/testcontainers-rs) to permit the latest version.
- [Release notes](https://github.com/testcontainers/testcontainers-rs/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-rs/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-rs@0.20.0...0.21.0)

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

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

Cargo.toml

@@ -12,7 +12,7 @@ keywords = ["testcontainers", "testing", "kafka", "redpanda", "integration-testi
 
 [dependencies]
 log = "0.4"
-testcontainers = { version = "0.20" }
+testcontainers = { version = "0.21" }
 
 [dev-dependencies]
 env_logger = "0.11"