Merge 1.20.6 into 1.21.1

[ZacSharp]

Includes

• Fix #litematica in obfuscated environments (fix litematica remapping error #4560)
• Don't load non-vanilla registries (Fix registry loading when custom registries present #4553)

[ZacSharp]

BTW is our version support policy still "latest minor of supported majors"?
If so I'll create a new pr directly to 1.21.3

[rfresh2]

in my opinion bug fixes should be merged to all affected versions.

but for new features you can reduce the set to something like:

• 1.20.1 -> active LTS of 1.20 for most mods
• 1.20.4 -> current 2b2t version
• 1.21.1 -> active LTS of 1.21 for most mods
• (optional) latest -> currently 1.21.4

all other versions including 1.19.4 and 1.20.6 are used by very few players.