revert unrequired change of making task abstract

Author: breskeby

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/release/ExtractCurrentVersionsTask.java

@@ -29,7 +29,7 @@
 
 import javax.inject.Inject;
 
-public abstract class ExtractCurrentVersionsTask extends AbstractVersionsTask {
+public class ExtractCurrentVersionsTask extends AbstractVersionsTask {
     private static final Logger LOGGER = Logging.getLogger(ExtractCurrentVersionsTask.class);
 
     private Path outputFile;