Code rework, Java 21 features can be used in core system (excluding addons)

[holgerfriedrich]

Java 21 language features can be used in core. Java 21 is delayed for add-ons (can be used in single plugins if owner wants it, but will not be rolled out over the whole repo until OH 5 is released in summer 2025).

Clean up the code and build system, port to Java 21, deprecations, etc. - this refers mainly to core, but can also include distro, webui.

• Java 21 language features Java 21 language features #4535
• Upgrade all maven plugins to latest possible Upgrade plugins #4536 Upgrade plugins #4537 Upgrade plugins for demo app openhab-distro#1719 Upgrade plugins openhab-distro#1716 Upgrade plugins openhab-addons#18042 Upgrade plugins openhab-addons#18037 Upgrade plugins openhab-webui#2972 [cometvisu] Upgrade openapi generator to 7.0.1 openhab-webui#2975 Upgrade plugins openhab-webui#2974
• Replace deprecated URL ctors Remove calls to deprecated URL constructor #4551
• Replace deprecated javax.annotation by jakarta.annotation https://docs.openrewrite.org/recipes/java/migrate/jakarta/javaxannotationmigrationtojakartaannotation

[holgerfriedrich]

@wborn have you already migrated annotations to Jakarta before? Any recommendations? Should we do this for the whole code base, including add-ons?