update pom to java 21.

[mishraomp]

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[mishraomp]

unfortunately, ecosystem is lagging, closing for the time being.
https://adoptium.net/en-GB/
25th of September 2023:We are awaiting access to the new Java 21's specification tests before formally releasing Temurin 21. In the meantime early access builds (untested and not for production use) can be downloaded from here or from our API.