Bump version number to 1.26.0 (#2236)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.26.0

Result:

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

Sources/GRPC/Version.swift

@@ -19,7 +19,7 @@ internal enum Version {
   internal static let major = 1
 
   /// The minor version.
-  internal static let minor = 25
+  internal static let minor = 26
 
   /// The patch version.
   internal static let patch = 0