macOS build process fixes and improvements

[Stereo]

I was doing this with Don-vip, but he tells me he's too busy to have a look at it this week, and to open a ticket.

​https://github.com/openstreetmap/josm/pull/68 improves the macOS build process:

• At Don-vip's suggestion, build .app bundles for both java 15 and 16-ea. In fact, we upload a .jar for every architecture and java variant.
• Infrastructure to allow josm.openstreetmap.de to trigger rebuilds for releases (#20148 and #20146 )
• Fix macOS dock icon resetting to default one. Fetches macOS icon over https (in case we're a jar), saves 77K in the jar. (#20133)
• Fix unreadable macOS tabs (#20075)
• Fix macOS tested build doesn't know it's tested (#20202)
• Better documentation of the macOS build process in the scripts, and better shell syntax
• Allow building of a JOSM.app without having signing keys installed - useful for local builds.

Also:

• Activate CodeQL code quality checks (not macOS-related; Don-vip and I were interested in testing it)

​https://github.com/openstreetmap/josm/pull/68.diff is the raw diff.

The diff without whitespace is more readable: ​https://github.com/openstreetmap/josm/pull/68/files?diff=split&w=1