Release v1.5

Widen version support to 1.8+, add proxy modes, rewrite the README

Targets Java 8 bytecode against spigot-api 1.16.5 with api-version 1.13, so
one jar now loads on everything from 1.8 to the latest release instead of
21-only builds.

- PurchaseManager reworked around claiming orders rather than reading them,
  so two backends behind a proxy cannot deliver the same order twice.
- serverInternalIp routes orders to the backend that owns a mode, for
  BungeeCord and Waterfall networks. Blank on a single server.
- deliverToOfflinePlayers holds an order until the buyer is online when the
  reward needs a live player.
- /quickshop test reports the API connection, with quickshop.test and
  quickshop.reload split out from quickshop.admin.
- Version 1.5.

README rewritten with the banner, install and config reference, and the
quick systems repo table. SETUP.md folded into it.