Update Android's rustc version 1.40.0->1.42.0

Test: cd external/rust/crates; mm
Bug: 152633654
Change-Id: Ic7cbc1869fdffa332033cecd4c4596775946d0e9
diff --git a/rust/config/global.go b/rust/config/global.go
index 690d83e..c0bd0f2 100644
--- a/rust/config/global.go
+++ b/rust/config/global.go
@@ -24,7 +24,7 @@
 var pctx = android.NewPackageContext("android/soong/rust/config")
 
 var (
-	RustDefaultVersion = "1.40.0"
+	RustDefaultVersion = "1.42.0"
 	RustDefaultBase    = "prebuilts/rust/"
 	DefaultEdition     = "2018"
 	Stdlibs            = []string{