Remove (most of the) advanced API

[Tim203]

Remove the following APIs, which will result in breaking changes in the API between Floodgate 2 and Floodgate 3:

• handshake handlers
• packet handlers
• injector addons
• floodgate logger

Handshake handlers

As of creating this issue the only API that already has been marked as deprecated already (for two years.) This system may be replaced with an event, but that's not certain yet.

Packet handlers

The idea was since Floodgate will be on many servers, we could make it a bit easier for people to use our existing injection so that they don't have to use other libraries. But instead of exposing the packets directly we should have a proper API to do what they want to do.

Injector addons

Injector addons are called when a channel has been injected by Floodgate (so when a player joins) and are called again when a player has left the server. Just like with packet handlers we should focus on creating an API that handles the logic for you and allows you to do what you want, without handling packets/channels directly.

Floodgate logger

To be honest, I don't know why this was ever part of the API.

Why

The first three items all have in common that they expose Netty logic. Netty should not be exposed in a library like this. Additionally with Floodgate 3 we want to be able to not just support Netty 4 but also Netty 5 and no Netty at all.