Bump version number to 1.21.1 (#1798)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.21.1

Result:

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

Sources/GRPC/Version.swift

@@ -22,7 +22,7 @@ internal enum Version {
   internal static let minor = 21
 
   /// The patch version.
-  internal static let patch = 0
+  internal static let patch = 1
 
   /// The version string.
   internal static let versionString = "\(major).\(minor).\(patch)"