[1.12.2] Revert built artifacts to UserDev 2

Currently, the 1.12.2 artifacts are built for UserDev 3, which is consumed by ForgeGradle 3 and newer. Discounting the various, unreported issues that have occurred with 1.12.2 on UserDev 3, for the sake of consistency, we should revert 1.12.2 back to UserDev 2.

ForgeGradle 7 can use the Minecraft Mavenizer to abstract away the implementation details between the various eras of Forge, while keeping compatibility with older build tools as well as 3rd-party build tools that rely on the older formats (python and pre-MCPConfig).

[!IMPORTANT] Please do not comment on 3rd-party build tools. You are free to do as you please with the software you build. But, we do not make them, nor will we endorse them.