Add CI support for Java 21

[dearrudam]

Last September 19th, the latest Java LTS, Java 21 was released and we should be able to run the source code against Java 21.

This issue will create a GitHub Action, CI, to run the source code against Java 21; this won't impact the code behavior. It is an excellent issue to the first contribution.

To-Do:

• [x] Create a GitHub Action that runs the Jakarta NoSQL source code against Java 21
• [x] Test the Action and ensure it correctly detects any compatibility issues
• [x] Update the README to document the new CI support for Java 21

[otaviojava]

That is nice!!! Thank you Max.