Migrated to API8

[masagameplay]

I migrated PacketGate to Sponge API8. I changed in this PR:

• Switched to Mojang mappings (Sponge's VanillaGradle)
• Pumped Gradle to 7.0.2
• Switched Reflections to Mixins
• Updated README
• Upgraded build.gradle to Kotlin DSL

I also introduced PacketGateAPI because API8 does not allow providing anything else than their implementations.

[Elikill58]

For publishing, there is multiple ways:

• Use github actions
• Auto-create a pom, to make it usage with jitpack
• Send jar to repo website that will generate pom file

Personnally, the github actions can be very useful for people. I personnally published the 0.2.0 version on my repo as release. I can setup it if you want, I have some experiences about it.

[masagameplay]

@Elikill58 feel free to do so. I could not figure out how to do so with CodeMC's docs