Remove finalization from RegistryKey

[basil]

Recent versions of Java have deprecated Object#finalize and marked it for removal. In this PR we are removing finalization from RegistryKey, which already implemented explicit termination via AutoCloseable. Unlike #8486, there were only a handful of usages throughout the ecosystem (I checked the cloudbees GitHub organization as well), and I audited each and every one to ensure it was already using the explicit terminator, either via try-with-resources or try-finally. Furthermore, all usages outside core were in extremely ancient plugins that are not relevant anymore. For core itself, SpotBugs would warn us if we introduced a new resource leak by forgetting to use try-with-resources. Weighing all of this against the complexity of reimplementing this (untested) functionality with the Cleaner API, it seemed best to simply dispense with this unused fallback mechanism rather than to port it to Java 9+.

Testing done

mvn clean verify -DskipTests

Proposed changelog entries

• Entry 1: Issue, human-readable text
• […]

Proposed upgrade guidelines

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[MarkEWaite]

Thanks!

[MarkEWaite]

This PR is now ready for merge. We will merge it after the release of the next weekly Jenkins version if there is no negative feedback. Merges to the master branch are intentionally blocked in preparation for the upcoming security release https://groups.google.com/g/jenkinsci-advisories/c/-RZfZDf9srA