Switch to Mojmappings

As Mojang is removing obfuscation from Minecraft Java, the Fabric team has made the decision to discontinue the Yarn project snd advises all mod developers to switch to Mojmappings in advance of the release of 1.21.11

Luckily, that's super easy to do—Loom has a migrateMappings Gradle task that I've already been relying on heavily to help port Better End and BCLib methods to LE's codebase (on that note: this will make future porting of BE features quite a bit simpler).

The biggest challenge here is going to be applying the remappings to the backports while keeping the Git history clean—and having the backports strictly ahead of the primary branch. But that's nothing a little soft resetting and interactive rebasing shouldn't be able to solve.