rust: Migrate default edition to 2021

Bug: 210475268
Test: Presubmit
Change-Id: I915a8779c032dc463bf76a1a41f545d29a7be044
diff --git a/rust/config/global.go b/rust/config/global.go
index 2821e31..bfb2d1f 100644
--- a/rust/config/global.go
+++ b/rust/config/global.go
@@ -26,7 +26,7 @@
 var (
 	RustDefaultVersion = "1.57.0"
 	RustDefaultBase    = "prebuilts/rust/"
-	DefaultEdition     = "2018"
+	DefaultEdition     = "2021"
 	Stdlibs            = []string{
 		"libstd",
 	}