Increase preload version to v6

We upgraded the dashboard image in kubernetes#9129, this will rebuild tarballs to include the latest version.

This should also fix a small performance regression, where start time was increased by 25% on kvm2, probably becaues we were waiting for the new dashboard image to download. [PR bot comment](kubernetes#9129 (comment))

pkg/minikube/download/preload.go

@@ -41,7 +41,7 @@ const (
 	// PreloadVersion is the current version of the preloaded tarball
 	//
 	// NOTE: You may need to bump this version up when upgrading auxiliary docker images
-	PreloadVersion = "v5"
+	PreloadVersion = "v6"
 	// PreloadBucket is the name of the GCS bucket where preloaded volume tarballs exist
 	PreloadBucket = "minikube-preloaded-volume-tarballs"
 )