Bump version to 1.18.0 (#1625)

Co-authored-by: George Barnett <[email protected]>

Author: gjcairo

Sources/GRPC/Version.swift

@@ -19,7 +19,7 @@ internal enum Version {
   internal static let major = 1
 
   /// The minor version.
-  internal static let minor = 17
+  internal static let minor = 18
 
   /// The patch version.
   internal static let patch = 0