Build with JDK 21

[javanegmond]

As outlined in #2886, here's a change that will build and pass all tests except e2e tests. I've not had time to run e2e tests, but they compile.

Note that this very eagerly changes all locations where a java version is referenced to 21. What level of support is desired for this library is up to the maintainers. I'll argue that at the very least it's gotta be newer than Java 8.

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[brendandburns]

I think it would be ok to take this as a separate branch, but we're not going to move to Java 21 on the mainline for now.

Even if we move forward, we're going to move forward to Java 11 or Java 17 not Java 21.

If you want to merge this into the mainline, the right option to move forward is to update the pom.xml to build two different versions of the library, not to replace the current version.

@yue9944882 wdyt?

[javanegmond]

Even if we move forward, we're going to move forward to Java 11 or Java 17 not Java 21.

Sounds absolutely fair to me. Java 11 still has good support from multiple providers until some time in 2024 IIRC.

If you want to merge this into the mainline, the right option to move forward is to update the pom.xml to build two different versions of the library, not to replace the current version.

All up to you guys. I just wanted to do a quick run of what effort is necessary to be able to build with Java 21, which turns out is not a lot, but it's not a freebie either.

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PR needs rebase.

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[brendandburns]

Closing this as we have moved to a different plan to move forward to JDK11