[FLINK-39098] Bump testcontainers to 1.21.4 for Docker API compatibility

[shivaam]

Purpose of the change

Bumps testcontainers from 1.17.2 to 1.21.4 to resolve Docker API compatibility issues (FLINK-39098 / #232).

Testcontainers 1.17.2 was released in June 2022 and does not support newer Docker API versions. Version 1.21.4 includes fixes for Docker Desktop compatibility and newer Docker Engine versions.

Verifying this change

This change is already covered by existing tests. All integration tests that exercise testcontainers pass:

• flink-connector-dynamodb — DynamoDB integration tests via LocalStack container
• flink-connector-aws-kinesis-streams — Kinesis Streams integration tests via LocalStack container
• flink-connector-aws-kinesis-firehose — Kinesis Firehose integration tests via LocalStack container
• flink-connector-sqs — SQS unit tests
• Full mvn verify passes across all 20 modules (tested with Docker Desktop 25.0.3 on macOS ARM64)

Significant changes

• Dependencies have been added or upgraded
• Public API has been changed (Public API is any class annotated with @Public(Evolving))
• Serializers have been changed
• New feature has been introduced
  • If yes, how is this documented? (not applicable / docs / JavaDocs / not documented)

[boring-cyborg]

Thanks for opening this pull request! Please check out our contributing guidelines. (https://flink.apache.org/contributing/how-to-contribute.html)

[gguptp]

I see @ferenc-csaky already attempted this here: https://github.com/apache/flink-connector-aws/pull/232/changes but that caused the workflows to fail, can we merge this PR with that and see how we can pass the CI?