Bump version number to 1.21.0 (grpc#1741)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.21.0

Result:

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

Author: glbrntt

Sources/GRPC/Version.swift

@@ -19,7 +19,7 @@ internal enum Version {
   internal static let major = 1
 
   /// The minor version.
-  internal static let minor = 20
+  internal static let minor = 21
 
   /// The patch version.
   internal static let patch = 0