Fix "illegal data type conversion to int" and change BANNED_PACKETS to HashSet

[4Benj]

Description

When DebugMode is set to MISSING (Not tested with ALL), an error (listed below) because PacketOpcodesUtil was expecting everything inside PacketOpcodes to be an int. However, it found a list causing the error below. This list was added in #582 and must have went unnoticed until now.

After a request from Melledy, I have also changed BANNED_PACKETS from a List to a HashSet

java.lang.IllegalArgumentException: Attempt to get java.util.List field "emu.grasscutter.net.packet.PacketOpcodes.BANNED_PACKETS" with illegal data type conversion to int
        at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.newGetIllegalArgumentException(UnsafeFieldAccessorImpl.java:69)
        at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.newGetIntIllegalArgumentException(UnsafeFieldAccessorImpl.java:132)
        at java.base/jdk.internal.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl.getInt(UnsafeQualifiedStaticObjectFieldAccessorImpl.java:58)
        at java.base/java.lang.reflect.Field.getInt(Field.java:601)
        at emu.grasscutter.net.packet.PacketOpcodesUtil.<clinit>(PacketOpcodesUtil.java:22)
        at emu.grasscutter.server.game.GameServerPacketHandler.handle(GameServerPacketHandler.java:98)
        at emu.grasscutter.server.game.GameSession.onMessage(GameSession.java:250)
        at emu.grasscutter.netty.KcpChannel.channelRead(KcpChannel.java:43)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
        at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
        at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
        at io.jpower.kcp.netty.UkcpServerChannel$UkcpServerUnsafe.read(UkcpServerChannel.java:603)
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:722)
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:658)
        at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:584)
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:496)
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:833)

Please carefully read the Contributing note and Code of conduct before making any pull requests.
And, Do not make a pull request to merge into stable unless it is a hotfix. Use the development branch instead.

Issues fixed by this PR

No related issues within the issue tracker (from what I could see anyway). However, look at the description for the issue that is being fixed by this PR.

Type of changes

• Bug fix
• New feature
• Enhancement
• Documentation

Checklist:

• My code follows the style guidelines of this project
• My pull request is unique and no other pull requests have been opened for these changes
• I have read the Contributing note and Code of conduct
• I am responsible for any copyright issues with my code if it occurs in the future.