Upgrade rust/crates/protobuf-codegen to 2.20.0

Test: make
Change-Id: I2aaabad83af5a304ad847943933dbe5a116ec575
diff --git a/METADATA b/METADATA
index a3beffb..5a52570 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/protobuf-codegen/protobuf-codegen-2.18.1.crate"
+    value: "https://static.crates.io/crates/protobuf-codegen/protobuf-codegen-2.20.0.crate"
   }
-  version: "2.18.1"
+  version: "2.20.0"
   license_type: NOTICE
   last_upgrade_date {
-    year: 2020
-    month: 11
-    day: 24
+    year: 2021
+    month: 1
+    day: 7
   }
 }