Bump org.testcontainers:cassandra from 1.20.6 to 1.21.0 (#300)

Bumps [org.testcontainers:cassandra](https://github.com/testcontainers/testcontainers-java) from 1.20.6 to 1.21.0.
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.20.6...1.21.0)

---
updated-dependencies:
- dependency-name: org.testcontainers:cassandra
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -9,7 +9,7 @@ simpleclient-dropwizard = { group = "io.prometheus", name = "simpleclient_dropwi
 assertj-core = { group = "org.assertj", name = "assertj-core", version = "3.27.3" }
 lz4-java = { group = "org.lz4", name = "lz4-java", version = "1.8.0" }
 mockito-core = { group = "org.mockito", name = "mockito-core", version = "5.17.0" }
-cassandra = { group = "org.testcontainers", name = "cassandra", version = "1.20.6" }
+cassandra = { group = "org.testcontainers", name = "cassandra", version = "1.21.0" }
 testng = { group = "org.testng", name = "testng", version = "7.11.0" }
 
 [plugins]