Prune a plugin's own stale staged jars in copyPlugin so server/plugins doesn't accumulate old versions

copyPlugin (treasury, business, treasury-api-plugin) used layout.projectDirectory.dir("../../server/plugins"), which from a top-level subproject climbs treasury → hibernia-economy → workspace and writes to C:\Users\rian\workspace\server\plugins — outside the repo, not the in-repo server/plugins the wiki/CLAUDE.md describe. As a Copy, it also never pruned old versions.

Fixed:

• Corrected the path to ../server/plugins (the documented in-repo dev server) in all three plugin builds.
• Before copying, delete only this artifact's previously-staged jars, matched on its own base name + a version digit (^treasury-\d.*\.jar$, so treasury never matches treasury-api-plugin). Sibling plugins (LuckPerms/WorldEdit/WorldGuard/ChestShop) and data folders are left untouched.

Verified: :treasury:copyPlugin now stages treasury-2.3.0 into the in-repo server, prunes treasury-2.2.1, and leaves every other jar and data folder intact.

Leftover: the old buggy path created a stray C:\Users\rian\workspace\server\plugins outside the repo (can be deleted).