Upgrade rust/crates/regex-syntax to 0.6.18

Test: None
Change-Id: Ibb0a5495aac0d848e661ddd70f729a76b949248f
diff --git a/METADATA b/METADATA
index e071c39..5bf8bef 100644
--- a/METADATA
+++ b/METADATA
@@ -9,11 +9,11 @@
     type: GIT
     value: "https://github.com/rust-lang/regex"
   }
-  version: "0.6.17"
+  version: "0.6.18"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 3
-    day: 31
+    month: 5
+    day: 28
   }
 }