Nox plugin

Maintenance release covering Minecraft 1.16 through 1.21.8. From 1.21.9 on, use version 1.1.0.

The wider range needed no code change: every Bukkit member the plugin references was verified to
exist in the 1.16.5, 1.17, 1.18, 1.18.2, 1.19, 1.19.4, 1.20.4, 1.20.6, 1.21, 1.21.4 and 1.21.8 APIs,
with `ServerListPingEvent` unchanged throughout. The jar is Java 8 bytecode, so it loads on every
Java version those releases require.

The 1.21.8 ceiling comes from ProtocolLib, which has not released support beyond it; without
ProtocolLib the version label, tooltip, hidden slots and fake online count cannot be applied.

### Fixed
- [x] fixed: Update checker reported the oldest release instead of the newest
- [x] fixed: Random MOTD never picked the last entry and threw on single-entry lists
- [x] fixed: `%banDate%` and `%banSource%` returned each other's value
- [x] fixed: Banned MOTD matched the reverse-DNS hostname, so IP bans rarely matched
- [x] fixed: Fake online count and hidden slots were never applied to the ping packet
- [x] fixed: Gradients dropped their last character
- [x] fixed: Single-colour gradient tags and very short gradients threw during the ping
- [x] fixed: Format codes inside a gradient were dropped (reference comparison instead of value)
- [x] fixed: Update notice overwrote the server-wide join message instead of messaging the operator
- [x] fixed: Join listener and `/nox checkUpdate` blocked the main thread on an HTTP call
- [x] fixed: `fakeOnlineFixedEnabled`, `fakeOnlineMultiply` and `fakeOnlineMultiplyEnabled` wrote to
      the wrong configuration key, so they changed an unrelated setting
- [x] fixed: Numeric command arguments threw instead of printing the usage
- [x] fixed: Distributed jar shipped bStats unrelocated, clashing with every other plugin using it
- [x] fixed: Version suffix in CI used the long-removed `CI_BUILD_REF_NAME` variable

### Changed
- [x] changed: ProtocolLib 4.7.0 to 5.3.0 - 4.7.0 was removed from every repository, and 5.3.0 still
      supports Minecraft 1.17 with the same API
- [x] changed: CI image from the retired `openjdk:16` to `azul/zulu-openjdk:16`

### Added
- [x] added: Verified support for Minecraft 1.18 through 1.21.8
- [x] added: `/nox` with no argument lists the commands available to the sender
- [x] added: Unit tests for version ordering and the hex/gradient parser