re-enable forge support

[pcal43]

As of 0.18.0+1.20.6, I will no longer be supporting the Forge build of fastback.

• The Forge version was an experiment that never left alpha. I don't use Forge personally and testing it and keeping the build working has become a chore. My time would be better spent elsewhere.

• It has been a useful exercise in that it has encouraged a cleaner separation of the non-loader-specific code (in the common module) and that will continue.

• If anyone in the community is interested in maintaining the Forge build, feel free to send a PR.

• FWIW, this is the build failure I'm currently getting try to upgrade to 1.20.6. I imagine it's not a difficult fix, but as I say, it's just not really something I want to spend my time on.

Mapping source name conflicts detected:
  net/minecraft/network/protocol/login/custom/DiscardedQueryPayload METHOD write ((Lnet/minecraft/network/FriendlyByteBuf;)V) -> [net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_295630_, net/minecraft/network/protocol/login/custom/DiscardedQueryPayload/m_294761_]
  net/minecraft/network/protocol/login/custom/CustomQueryPayload METHOD write ((Lnet/minecraft/network/FriendlyByteBuf;)V) -> [net/minecraft/network/protocol/login/custom/CustomQueryPayload/m_294761_, net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_295630_]
  net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload METHOD write ((Lnet/minecraft/network/FriendlyByteBuf;)V) -> [net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_295630_, net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_294761_]
  net/minecraftforge/network/ForgePayload METHOD write ((Lnet/minecraft/network/FriendlyByteBuf;)V) -> [net/minecraftforge/network/ForgePayload/m_294761_, net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_295630_]
  net/minecraft/network/protocol/login/custom/DiscardedQueryAnswerPayload METHOD write ((Lnet/minecraft/network/FriendlyByteBuf;)V) -> [net/minecraft/network/protocol/login/custom/CustomQueryAnswerPayload/m_295630_, net/minecraft/network/protocol/login/custom/DiscardedQueryAnswerPayload/m_294761_]