Update Java to current LTS

Domino 14 has Java 17, which is good, but the timing of things meant that that's no longer the current LTS. It makes sense that Domino wouldn't move to 21 for in R14, but it'll be important to avoid another situation like what happened with Java 6 and 8, where Domino lags far behind current Java versions up until the point where it's completely untenable.

The next Jakarta EE version targets 21 as a minimum, and I expect other projects to follow suit: it seems that it's becoming more culturally normal for frameworks and libraries to be fairly aggressive about bumping up JVM requirements. Domino should follow suit, and ideally have a known planned timeline.