Upgrade rust/crates/regex-syntax to 0.6.21

Test: make
Change-Id: I8c34d16c9a7049044418c71b091f19218c72526d
diff --git a/METADATA b/METADATA
index 70c34e5..dda6e41 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/regex-syntax/regex-syntax-0.6.20.crate"
+    value: "https://static.crates.io/crates/regex-syntax/regex-syntax-0.6.21.crate"
   }
-  version: "0.6.20"
+  version: "0.6.21"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 10
-    day: 26
+    month: 11
+    day: 2
   }
 }