[FLINK-39099] Update testcontainers version to 1.21.4

[ferenc-csaky]

Trivial dependency bump to avoid Docker client too old errors:

org.testcontainers.dockerclient.DockerClientProviderStrategy [] -     UnixSocketClientProviderStrategy: failed with exception BadRequestException (Status 400: {"message":"client version 1.32 is too old. Minimum supported API version is 1.44, please upgrade your client to a newer version"}

This can affect CI (GitHub) and downstream connector projects.

[flinkbot]

CI report:

• 647147a Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[gaborgsomogyi]

Assuming you can kick it to green ci :)

[davidradl]

@ferenc-csaky fyi I see a duplicate #27606

[davidradl]

@flinkbot run azure

[ferenc-csaky]

The CI failure is unrelated and there is already a PR to cover that: #27619

Hence, I'll merge this if no objections until EOD today.

[pnowojski]

Hey @ferenc-csaky a friendly reminder for the future. Next time please do not create duplicate PRs and do not supersede identical PRs that were created earlier. Especially when this was pointed out to you.

[ferenc-csaky]

Now that @MartijnVisser confirmed on #27619 the cause of the CI failure (funny enough, same cause, but it's cause we bumped Azure runner images as well), I'm merging this.

[ferenc-csaky]

@pnowojski I'm happy to merge the other one, there was just more context here and also on the Jira, so I opted for trackability. But it's a fair point.

[pnowojski]

Yeah, it's a minor thing in this case, and I agree that I would be on the fence myself given that this PR has a jira ticket attached and the other doesn't, so it's neither a big nor a clear cut issue in this case. But we still need to be very careful when it comes to preserving and akcnolweding the authorship. Thank you for your understanding.