Revert "Drop `@Input` from `getData()`"

This reverts commit a9e50d67dd as
we do have some undeclared inputs.

See gh-40572

buildSrc/src/main/java/org/springframework/boot/build/antora/GenerateAntoraPlaybook.java

@@ -93,7 +93,8 @@ public abstract class GenerateAntoraPlaybook extends DefaultTask {
 		}
 	}
 
-	private Map<String, Object> getData() throws IOException {
+	@Input
+	final Map<String, Object> getData() throws IOException {
 		Map<String, Object> data = loadPlaybookTemplate();
 		addExtensions(data);
 		addSources(data);