Fix missing maven central in build.gradle

Broke for someone trying to setup a workspace by it not being absent. Not sure why it worked for me before, but no harm having it (we notably have it on the 1.20 Mantle branch)

build.gradle

@@ -15,6 +15,7 @@ plugins {
 }
 
 repositories {
+    mavenCentral()
     maven {
         // Our maven
         name "Prog's Maven"