Migrate to CompletableFuture / HttpClient usage, misc changes

[TheRealRyGuy]

This is most absolutely breaking backwards compat, but I believe it's worth it
This migrates off of Java 8 fully, making use of Java 11's HttpClient (if you want to maintain Java 8 compat, easy enough to just use a different library)
Changes list

• Adjusted all fields to be private, and where necessary, final
• Remove redundant git submodules step from building
• Migrated off of legacy Java http connections, making use of their Java HttpClient
• All API based methods now are wrapped around a CompletableFuture, as they are all ran async
• Made some misc changes

Fully aware that this would break every mclogs based plugin - more than happy to PR relevant changes if this is approved and implemented

[CLAassistant]

All committers have signed the CLA.

[TheRealRyGuy]

Any update on this?

[JulianVennen]

Hi, thanks for reminding me, I forgot about this PR.

I wanted to keep Java 8 support because old forge versions don't support newer Java versions, but supporting ancient Minecraft versions is annoying in many ways, so it's probably fine to drop them.

[JulianVennen]

Thank you for your contribution, 4.0.0 was released and will be on maven central as soon as sonatype syncs it