[Release] Update versions for 12.6.0

ReleaseTooling/Sources/FirebaseManifest/FirebaseManifest.swift

@@ -21,7 +21,7 @@ import Foundation
 /// The version and releasing fields of the non-Firebase pods should be reviewed every release.
 /// The array should be ordered so that any pod's dependencies precede it in the list.
 public let shared = Manifest(
-  version: "12.5.0",
+  version: "12.6.0",
   pods: [
     Pod("FirebaseSharedSwift"),
     Pod("FirebaseCoreInternal"),