Pin FG5 version because its maintainers cannot be trusted

Versions after 5.0.5 emit tsrg2 instead of tsrg, breaking anything which reads
those files (e.g. Mixin and the preprocessor), later versions even bump the
required Gradle version.

build.gradle

@@ -36,7 +36,7 @@ buildscript {
         if (fabric) {
             classpath 'fabric-loom:fabric-loom.gradle.plugin:0.8-SNAPSHOT'
         } else if (mcVersion >= 11400) {
-            classpath('net.minecraftforge.gradle:ForgeGradle:5.+'){
+            classpath('net.minecraftforge.gradle:ForgeGradle:5.0.5') { // the FG people still haven't learned to not do breaking changes
                 exclude group: 'trove', module: 'trove' // preprocessor/idea requires more recent one
             }
         } else if (mcVersion >= 10800) {