Bump version number to 1.4.1-async-await.2 (#1291)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.4.1-async-await.2

Result:

The version in the default user-agent string will match the released
version.

Sources/GRPC/Version.swift

@@ -25,5 +25,5 @@ internal enum Version {
   internal static let patch = 0
 
   /// The version string.
-  internal static let versionString = "\(major).\(minor).\(patch)"
+  internal static let versionString = "\(major).\(minor).\(patch)-async-await.2"
 }