java: fix Java21 Warnings

[justadreamer]

See the warnings documented:

• https://github.com/postindustria-tech/pipeline-java/blob/j21-warnings/java21-warnings.md
• https://github.com/postindustria-tech/device-detection-java/blob/j21-warnings/java21-warnings.md

Once these are fixed we need to enable "-Werror" for Java - not exactly this setting, but whatever the flag is for javac to treat warnings as errors.