core: Fix use of incorrect project import

Author: zml2008

src/main/java/net/kyori/mammoth/GradleCompat.java

@@ -23,7 +23,7 @@
  */
 package net.kyori.mammoth;
 
-import org.apache.tools.ant.Project;
+import org.gradle.api.Project;
 
 final class GradleCompat {
   static final boolean HAS_CONVENTION;