Increase preload version to v6

[priyawadhwa]

We upgraded the dashboard image in #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 because preload failed and we were waiting for the new dashboard image to download. PR bot comment

[k8s-ci-robot]

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[afbjorklund]

I think that once one image is missing from the preload, it gives up on all the other ones too.

Actually it only does a retrieve, which re-downloads the manifest - but the layers are still there

[priyawadhwa]

/ok-to-test

[tstromberg]

@priyawadhwa - can you approve #9151 first?

[minikube-pr-bot]

kvm2 Driver
Times for minikube: 86.5s 105.4s 95.0s
Average time for minikube: 95.7s

Times for Minikube (PR 9159): 114.2s 104.3s 100.5s
Average time for Minikube (PR 9159): 106.3s

Averages Time Per Log

+--------------------------------+----------+--------------------+
|              LOG               | MINIKUBE | MINIKUBE (PR 9159) |
+--------------------------------+----------+--------------------+
| * minikube v1.12.3 on Debian   | 0.2s     | 0.1s               |
|                           9.11 |          |                    |
| * Using the kvm2 driver based  | 0.0s     | 0.0s               |
| on user configuration          |          |                    |
| * Starting control plane node  | 0.0s     | 0.0s               |
| minikube in cluster minikube   |          |                    |
| * Creating kvm2 VM (CPUs=2,    | 43.7s    | 20.3s              |
| Memory=3700MB, Disk=20000MB)   |          |                    |
| ...                            |          |                    |
| * Preparing Kubernetes v1.19.0 | 49.3s    | 82.3s              |
| on Docker 19.03.12 ...         |          |                    |
| * Verifying Kubernetes         | 1.7s     | 1.9s               |
| components...                  |          |                    |
| * Enabled addons:              | 0.5s     | 0.1s               |
| default-storageclass,          |          |                    |
| storage-provisioner            |          |                    |
| * Done! kubectl is now         | 0.2s     | 1.5s               |
| configured to use "minikube"   |          |                    |
|                                | 0.0s     | 0.0s               |
+--------------------------------+----------+--------------------+

docker Driver
Times for minikube: 65.1s 55.4s 53.2s
Average time for minikube: 57.9s

Times for Minikube (PR 9159): 59.1s 58.3s 59.5s
Average time for Minikube (PR 9159): 59.0s

Averages Time Per Log

+--------------------------------+----------+--------------------+
|              LOG               | MINIKUBE | MINIKUBE (PR 9159) |
+--------------------------------+----------+--------------------+
| * minikube v1.12.3 on Debian   | 0.3s     | 0.1s               |
|                           9.11 |          |                    |
| * Using the docker driver      | 0.0s     | 0.0s               |
| based on user configuration    |          |                    |
|                                | 0.0s     | 0.0s               |
|                                |          |                    |
| * Starting control plane node  |          |                    |
| minikube in cluster minikube   |          |                    |
| * Creating docker container    |          |                    |
| (CPUs=2, Memory=3700MB) ...    |          |                    |
| * Preparing Kubernetes v1.19.0 |          |                    |
| on Docker 19.03.8 ...          |          |                    |
| * Verifying Kubernetes         |          |                    |
| components...                  |          |                    |
| * Enabled addons:              |          |                    |
| default-storageclass,          |          |                    |
| storage-provisioner            |          |                    |
| * Done! kubectl is now         |          |                    |
| configured to use "minikube"   |          |                    |
|                                |          |                    |
+--------------------------------+----------+--------------------+