Fix compilation in FileExtensionMatchers

Java 8 can not infer types as well as Java 11 does. This means a
backport of some code that relied on Java 11's superior abilities to
infer types caused Java 8's head to explode trying to infer the same
type in 7.x. This commit addresses this by giving Java 8 the types that
it needs.

qa/os/src/test/java/org/elasticsearch/packaging/util/FileExistenceMatchers.java

@@ -72,7 +72,7 @@ public class FileExistenceMatchers {
             if (Files.isDirectory(path)) {
                 mismatchDescription.appendText("path " + path + " is a directory with contents\n");
                 try {
-                    Files.walkFileTree(path, new SimpleFileVisitor<>() {
+                    Files.walkFileTree(path, new SimpleFileVisitor<Path>() {
 
                         @Override
                         public FileVisitResult visitFile(final Path file, final BasicFileAttributes attrs) throws IOException {