Custom Mob Spawns

Changes in this version:

• Removed support for 1.19.3, 1.19.4, 1.20, 1.20.1, 1.20.2, 1.20.3, 1.20.4, 1.20.5, and 1.20.6 due to changes in Fabric's "Identifier".
• Added 1.21 support (also works with 1.21.1).

Behind the scenes:

• Updated Cloth Config minimum version from 11.0.99 to 15.0.127 (Aka the earliest version for 1.21)
• Updated Mod Menu minimum version from 7.0.0 to 11.0.0-beta.1 (Aka the earliest version for 1.21)
• Updated Fabric API minimum version from 0.68.1 to 0.100.1 (Aka the earliest version for 1.21)
• Updated default game version of the mod from 1.20 to 1.21