[patch] Update linux packaging files

[gaben]

I noticed the files used on Linux distros are outdated by years and some of the scripts recognizing Java runtime too. Also while doing so, realized a discrepancy in the validator CLI, which is also fixed.

[gaben]

Two patches attached:

• 1-docs-linux-manpages.patch updates the Linux packaging. The manpages now match the current --help output: they document the render, project and validate commands, the launcher environment variables (JAVACMD, JAVAFX_HOME, JOSM_MODULEPATH, JOSM_PATH) and have a few more examples.

The launchers know about Java 25-28, the Debian package description and /etc/default files are refreshed, and the unused Docker files are removed. The runjosm help now also mentions the validate command.

• 2-validator-set.patch fixes josm validate --set key=value, which crashed with an AssertionError because the option was never handled. It now uses the same parsing as runjosm and can be given multiple times. Also includes a unit test.