Paper 1.20.6/1.21 event warning when using brigadier

When registering commands on Paper 1.20.6 (https://github.com/PaperMC/Paper/commit/e41d44f) and 1.21 (https://github.com/PaperMC/Paper/commit/2df432f) with the unstable `brigadier` API enabled, the following warning is printed:
```
[02:54:19 WARN]: [Plugin] "Plugin vx.x.x" has registered a listener for com.destroystokyo.paper.event.brigadier.CommandRegisteredEvent on method "public void my.package.libs.co.aikar.commands.PaperBrigadierManager.onCommandRegister(com.destroystokyo.paper.event.brigadier.CommandRegisteredEvent)", but the event is Deprecated. "This event has been superseded by the Commands API and will be removed in a future release. Listen to LifecycleEvents.COMMANDS instead."; please notify the authors [Insprill].
```

Minimal repro:
```java
public final class Main extends JavaPlugin {
@Override
public void onEnable() {
PaperCommandManager commandManager = new PaperCommandManager(this);
commandManager.enableUnstableAPI("brigadier");
}
}
```