Switch the root build scripts to Kotlin and limit parallel remaps

- settings.gradle, stonecutter.gradle and the root build.gradle become
  .kts. Loom's version for the Stonecutter versions moves to the root
  gradle.properties (loom_version), read by pluginManagement, since the
  Kotlin plugins block can't read properties. The old mc-<v> projects keep
  their Groovy scripts until they're folded in.
- buildAll ran all eight remapJar tasks at once. Each loads a Minecraft
  classpath and remaps a ~50 MB jar, which doesn't fit in the 3 GB daemon
  heap: a cold build stalled in garbage collection for 40+ minutes, and
  earlier runs failed. A shared build service now lets two remaps run
  together; a cold buildAll takes about 2 minutes.
- port.sh and VERSION_GUIDE follow the .kts file names.

The 1.21.11 jar is the same as with the Groovy scripts.

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

versions/1.21.11/gradle.properties

@@ -10,7 +10,6 @@ org.gradle.configuration-cache=false
 minecraft_version=1.21.11
 yarn_mappings=1.21.11+build.4
 loader_version=0.19.0
-loom_version=1.17.21
 
 fabric_api_version=0.141.3+1.21.11