Migrate Forge build setup to ForgeGradle 7

[Jonathing]

:wave: Good afternoon!

I'm Jonathan. I'm from the Forge team and I am the developer of ForgeGradle 7, Forge's new UserDev toolchain handler. As part of the stabilization of ForgeGradle 7 from beta to release, I am taking the time to reach out to as many multi-loader templates I know to personally aid in the migration process. Please let me know of any others you know of, as I'm not well-versed on them.

I'm marking this PR as a draft for now since ForgeGradle 7 is not yet fully released (also I am currently using Maven Local for testing), but as of right now, it is fully functional. There are a couple of things I don't quite grasp with your template, but I've done my best to keep the layout as close to the original as possible. The biggest change (for modders) from ForgeGradle 6 to 7 is the complete API breakage, which is what this PR is meant to account for.

I'd like to use this pull request as a thread for open discussion on ForgeGradle 7. I'm looking forward to hearing your thoughts on how it works with the template, as well as any comments, criticisms, and suggestions you may have about the plugin. Cheers, and I look forward to working with you soon.