JOSM Nexus 2 repository shutdown

JOSM's Nexus 2 is already partially migrated to Nexus 3, giving the new store a new URL endpoint. For more details, see the migration ticket: https://josm.openstreetmap.de/ticket/23716

I think the JAR download functions needs to be updated to point to the new location: https://gitlab.com/JOSM/gradle-josm-plugin/-/blob/11a651950b0b4c018ccdd2949b9220c4fbf71f4c/plugin/src/main/kotlin/org/openstreetmap/josm/gradle/plugin/util/Urls.kt#L32-36

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information