Upgrade rust/crates/structopt to 0.3.21

Test: make
Change-Id: I8b7f385accd0609f9b56cc77c55af99ca40c5241
diff --git a/METADATA b/METADATA
index b4901c8..df2195b 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/structopt/structopt-0.3.20.crate"
+    value: "https://static.crates.io/crates/structopt/structopt-0.3.21.crate"
   }
-  version: "0.3.20"
+  version: "0.3.21"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 10
-    day: 26
+    month: 11
+    day: 30
   }
 }