Migrate from Apache Commons StringUtils#join to Java Platform String#join (#5992)

core/src/main/java/hudson/FilePath.java

@@ -1639,7 +1639,7 @@ public final class FilePath implements SerializableOnlyOverRemoting {
             } else {
                 s = new String[]{prefix, suffix};
             }
-            String name = StringUtils.join(s, ".");
+            String name = String.join(".", s);
             return new FilePath(this, act(new CreateTempDir(name)));
         } catch (IOException e) {
             throw new IOException("Failed to create a temp directory on "+remote,e);