Fix handshake error with Geyser ping passthrough on Velocity

[MasonooN]

Summary

Fixes spurious handshake/manipulation errors on Velocity when Geyser's ping
passthrough is enabled. Geyser pings arrive as a GeyserInboundConnection,
which isn't a real player connection the plugin can manipulate, but the
handler previously treated any unrecognized connection as LEGACY and tried
to disconnect/manipulate it, throwing errors.

Changes

• Add an UNKNOWN connection type in VelocityPlayer. Connections are now
  only classified as LEGACY when they actually match
  HandshakeSessionHandler$LegacyInboundConnection; anything unrecognized
  (e.g. Geyser's ping passthrough connection) is marked UNKNOWN instead of
  being misidentified as legacy.
• Skip UNKNOWN connections in VelocityHandshakeHandler.handleEvent(),
  they're ignored silently rather than disconnected or manipulated.
• Add the CodeMC Maven repository to build.gradle so net.md-5:bungeecord-api
  resolves (the Spigot snapshot repo now blocks automated builds).
• Minor: velocity-plugin.json reformatted by the build's updateVersion task.

Testing

• Builds successfully (gradle build, JDK 21).
• Verify on a Velocity proxy with Geyser ping passthrough enabled that the
  handshake errors no longer appear and normal Java/Bedrock connections still work.