Replace home-made legacy hover event serializer with Adventure's implementation

This technically can break backwards compatibility, but this seems to be very unlikely to be the practice in reality. (The Velocity implementation probably wasn't correct, anyway.)

proxy/build.gradle.kts

@@ -127,7 +127,7 @@ dependencies {
     runtimeOnly(libs.disruptor)
     implementation(libs.fastutil)
     implementation(platform(libs.adventure.bom))
-    implementation("net.kyori:adventure-nbt")
+    implementation(libs.adventure.text.serializer.json.legacy.impl)
     implementation(libs.adventure.facet)
     implementation(libs.completablefutures)
     implementation(libs.nightconfig)