Lower required Java level to 21

[jperedadnr]

Issue

Fixes #721

Progress

• Change must not contain extraneous whitespace
• License header year is updated, if required
• The PR name must follow the pre-defined format
• Verify the contributor has signed Gluon Individual Contributor License Agreement (CLA)

[johanvos]

Might be good to replace the commit message to "lower required Java level to 21"
(avoiding worries from people who think that they can't use Java 23)

[Oliver-Loeffler]

Might be good to replace the commit message to "lower required Java level to 21" (avoiding worries from people who think that they can't use Java 23)

We can make a statement about this setup in README.md Requisites Section. @jperedadnr Could you please add this change? When adding this, could you please attempt to rephrase the commit message?

[Oliver-Loeffler]

Could you please update README.md Requisites Section accordingly?

[jperedadnr]

I've updated the readme, that now says Java 21 or later, so that is good enough I guess.

[jperedadnr]

The commit message will take the PR's title once squashed and merged