Update minimum TypeScript version

Updated from 2.7 to 3.7

Author: andygup

4.x/typescript/README.md

@@ -9,7 +9,7 @@ Currently, due to limitations in TypeScript, the APIs [autocasting](https://deve
 
 ## Requirements
 
-* [TypeScript][2] 2.7+
+* [TypeScript][2] 3.7+
 
 ## Resources