Update to Java 21 and Gradle 8.5#9

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jra-se wants to merge 4 commits intodevfrom

Commits on Oct 28, 2025

Commits on Jan 16, 2026

Merge branch 'dev' into java21
jra-se
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Commits on Mar 4, 2026

Update doc to new Java and Gradle version
Hellwig-SE
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