Set ForgeGradle version to 6.0.6

ForgeGradle version will need to be manually updated, which should be
better than suddenly compiling with a different version of ForgeGradle
when compiling in the future, which may break the "reproducibility" in
reproducible builds.

build.gradle

@@ -6,7 +6,7 @@ buildscript {
         gradlePluginPortal()
     }
     dependencies {
-        classpath group: 'net.minecraftforge.gradle', name: 'ForgeGradle', version: '6.0.+', changing: true
+        classpath group: 'net.minecraftforge.gradle', name: 'ForgeGradle', version: '6.0.6'
         classpath 'com.github.johnrengelman:shadow:8.1.1'
     }
 }