NIAD-3347: Update `Java` to version `21`#294

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MartinWheelerMT merged 7 commits intodevelopfrom

niad-3347-update-java-to-version-21

May 19, 2025

Commits on May 16, 2025

* Add updated handleExceptionInternal override method to `OperationOutcomeExceptionHandler`, this now uses `HttpStatusCode` as so the expected response method is adjusted slightly.
MartinWheelerMT
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* Update documentation to reflect change to Java 21
MartinWheelerMT
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Commits on May 19, 2025

* Change gradle packages from `gradle:8.5-jdk21` to `gradle:jdk21`
MartinWheelerMT
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