Add missing @since tag on FileTreeIterator#getLastModifiedInstant

Change-Id: I809399e6a71e0079d2f0007b0d3f00b531d451bb
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/FileTreeIterator.java

@@ -412,6 +412,9 @@ public class FileTreeIterator extends WorkingTreeIterator {
 			return attributes.getLastModifiedInstant().toEpochMilli();
 		}
 
+		/**
+		 * @since 5.1.9
+		 */
 		@Override
 		public Instant getLastModifiedInstant() {
 			return attributes.getLastModifiedInstant();