[Hotfix] Update testcontainers to 1.21.4

[eschcam]

What is the purpose of the change

Testcontainers 1.20.2 has compatibility issues with newer versions of docker and can cause an Previous attempts to find a Docker environment failed. Will not retry. error to occur when running tests locally.

Brief change log

• Update testcontainers to 1.21.4

Verifying this change

Passes local tests

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): yes
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f106af2 Azure: FAILURE

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[eschcam]

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