Build 1.21.6 with Stonecutter from the shared src/

1.21.6's copy only differed from 1.21.7's in RenderScopeImpl: item
rendering uses ItemRenderState, which 1.21.7 renamed to
KeyedItemRenderState. That's behind a version comment; 1.21.6 keeps its
older loader_version (0.18.5) in its gradle.properties.

Moving a version over leaves the old mc-<v> project's compiled classes in
the shared build/ folder, so a removed class (1.21.6's dead NameTagMixin)
still got packaged until a clean. VERSION_GUIDE now says to run
:<v>:clean once after moving a version.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

settings.gradle.kts

@@ -24,7 +24,7 @@ include("common")
 
 // Versions built by Stonecutter from the shared src/ (see stonecutter.gradle.kts). Each one's
 // versions/<v>/gradle.properties holds its Minecraft, Yarn and Fabric API versions.
-val stonecutterVersions = listOf("1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
+val stonecutterVersions = listOf("1.21.6", "1.21.7", "1.21.8", "1.21.9", "1.21.10", "1.21.11")
 
 stonecutter {
     create(rootProject) {