* Fix Gradle deprecations
By using test suites, we explicitely configure the relevant dependencies
on the test sourceset. This is not done by merely configuring the test task.
* Switch to maintained version of Shadow
* Update to Gradle 8.14.2
* Update Adventure to 4.16.0
Also migrated shadow from `com.github.johnrengelman.shadow` to `io.github.goooler.shadow`, which fixes the project compilation using Java 21 and Gradle 8.6
* Update Gradle to 8.6 release
* Delay switch to new server until after JoinGame is sent.
Unfortunately, in some cases (especially vanilla Minecraft) some login
disconnects are sent after ServerLoginSuccess but before JoinGame.
We've been using ServerLoginSuccess but it has caused too many problems.
Now Velocity will switch to a stripped-down version of the play session
handler until JoinGame is received. This handler does very little by
itself: it simply forwards plugin messages (for Forge) and waits for the
JoinGame packet from the server.
This is an initial version: only vanilla Minecraft 1.12.2 was tested.
However this is the way Waterfall without entity rewriting does server
switches (which, in turn, is inherited from BungeeCord).
* Move to Gradle 5 and Error Prone.
