[FLINK-7186] Activate checkstyle flink-java/sampling

[dawidwys]

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[zentol]

+1.

I'm wondering if we should beef up the tools/maven/suppressions-java.xml file to have suppressions per package like flink-runtime, and remove an entry in each PR. Just as a safety precaution in case we don't get this done in a timely manner, to prevent new violations being added.

[zentol]

merging.