Update to 1.21.4

[TheDeathlyCow]

Frostiful will be updated to 1.21.4 after the 1.21.3 update is complete (#130). This version will be fully supported alongside 1.21.1 until a new major version/game drop is released for vanilla.

Version numbering will match the 1.21.1 version, but with the mc version appended. For example, frostiful-2.0.0+1.21.1 and frostiful-2.0.0+1.21.4 will have the same features, but will be available for different game versions.