Add 1.21.1 to supported versions of 0.4.7 on Modrinth and CurseForge

https://modrinth.com/mod/isometric-renders/version/0.4.7+1.21
https://www.curseforge.com/minecraft/mc-mods/isometric-renders/files/5432902

1.21.1 was just a hotfix and your FMJ's Minecraft dependency is very permissive, which isn't so much of a great thing but it allows you to modify the Modrinth and CurseForge metadata