Update dependency pmd to v7 (master)

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
pmd (source)	5.8.1 -> 7.7.0

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[Donnerbart]

We can make PMD 7 working, even with the outdated Gradle version. But we get a ton of new violations: 347 PMD rule violations were found. See the report at: file:///home/runner/work/hivemq-mqtt-client/hivemq-mqtt-client/build/reports/pmd/main.html

So we either would need to suppress those or do a quite substantial refactoring of the client.