"Kubernetes <version> is now available" upgrade prompt recommends a flag that does not work

[tetchel]

Steps to reproduce the issue:

1. Minikube lets me know that a new Kubernetes version is available:

🆕  Kubernetes 1.18.2 is now available. If you would like to upgrade, specify: --kubernetes-version=1.18.2

2. I specify that flag with my next minikube start
3. The start fails because the flag that was recommended does not work because the version has to start with a "v"
4. I tried it again with the "v" and it worked.

The upgrade prompt flag should have the right version syntax, with the "v", that's all.

🆕  Kubernetes 1.18.2 is now available. If you would like to upgrade, specify: --kubernetes-version=v1.18.2

Full output of minikube start command used, if not already included:

[ /Users/tim ] 50 $ minikube start
😄  minikube v1.10.1 on Darwin 10.15.4
✨  Using the hyperkit driver based on existing profile
🆕  Kubernetes 1.18.2 is now available. If you would like to upgrade, specify: --kubernetes-version=1.18.2
🆕  Kubernetes 1.18.2 is now available. If you would like to upgrade, specify: --kubernetes-version=1.18.2
👍  Starting control plane node minikube in cluster minikube
🔄  Restarting existing hyperkit VM for "minikube" ...
^C
[ /Users/tim ] 50 $ minikube start --kubernetes-version=1.18.2
😄  minikube v1.10.1 on Darwin 10.15.4
✨  Using the hyperkit driver based on existing profile
👍  Starting control plane node minikube in cluster minikube
💾  Downloading Kubernetes v1.18.2 preload ...
    > preloaded-images-k8s-v3-v1.18.2-docker-overlay2-amd64.tar.lz4: 525.43 MiB
🔄  Restarting existing hyperkit VM for "minikube" ...
🐳  Preparing Kubernetes v1.18.2 on Docker 19.03.8 ...
❌  Unable to load cached images: loading cached images: stat /Users/tim/.minikube/cache/images/k8s.gcr.io/kube-proxy_v1.18.2: no such file or directory

❌  [KUBERNETES_VERSION_MISSING_V] Failed to update cluster updating node: generating kubeadm cfg: parsing Kubernetes version: strconv.ParseUint: parsing "": invalid syntax
💡  Suggestion: Check that your --kubernetes-version has a leading 'v'. For example: 'v1.1.14'
[ /Users/tim ] 51 $ minikube start --kubernetes-version=v1.18.2
😄  minikube v1.10.1 on Darwin 10.15.4
✨  Using the hyperkit driver based on existing profile
👍  Starting control plane node minikube in cluster minikube
🏃  Updating the running hyperkit "minikube" VM ...
🐳  Preparing Kubernetes v1.18.2 on Docker 19.03.8 ...
🌟  Enabled addons: dashboard, default-storageclass, ingress, storage-provisioner
🏄  Done! kubectl is now configured to use "minikube"

[afbjorklund]

Duplicate, #8114

Will be fixed in next release, and workaround above should be enough in the meantime.