Update default kubernetes version to 1.18.2

[floryut]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Update default kubernetes version to 1.18.2

Which issue(s) this PR fixes:
Fixes #5859

Special notes for your reviewer:
Let's see what CI is thinking and not wait as long as we did with 2.13 :)

Does this PR introduce a user-facing change?:

Update default kubernetes version to 1.18.2

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[Miouge1]

Any kubeadm changes to consider as well?

[floryut]

Any kubeadm changes to consider as well?

Well I guess we have at least some kind of deprecated thing

TASK [kubernetes/node : sets kubeadm api version to v1beta1] *******************
�[1;30mtask path: /builds/kargo-ci/kubernetes-sigs-kubespray/roles/kubernetes/node/tasks/kubelet.yml:13�[0m
�[1;30m�[0mSaturday 02 May 2020 08:55:58 +0000 (0:00:01.215) 0:13:28.839 **********
�[0;32mok: [instance-2] => {"ansible_facts": {"kubeadmConfig_api_version": "v1beta1"}, "changed": false}�[0m
�[0;32m�[0m�[0;32mok: [instance-1] => {"ansible_facts": {"kubeadmConfig_api_version": "v1beta1"}, "changed": false}�[0m
�[0;32m�[0m

with those warning

your configuration file uses a deprecated API spec: "kubeadm.k8s.io/v1beta1". Please use 'kubeadm config migrate --old-config old.yaml --new-config new.yaml

[Miouge1]

I started all optional CI jobs to see if anything pops up there as well (except ubuntu 20.04 which is supposed to fail anyway).

[Miouge1]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, Miouge1

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[EppO]

Any kubeadm changes to consider as well?

Well I guess we have at least some kind of deprecated thing

TASK [kubernetes/node : sets kubeadm api version to v1beta1] *******************
�[1;30mtask path: /builds/kargo-ci/kubernetes-sigs-kubespray/roles/kubernetes/node/tasks/kubelet.yml:13�[0m
�[1;30m�[0mSaturday 02 May 2020 08:55:58 +0000 (0:00:01.215) 0:13:28.839 **********
�[0;32mok: [instance-2] => {"ansible_facts": {"kubeadmConfig_api_version": "v1beta1"}, "changed": false}�[0m
�[0;32m�[0m�[0;32mok: [instance-1] => {"ansible_facts": {"kubeadmConfig_api_version": "v1beta1"}, "changed": false}�[0m
�[0;32m�[0m

with those warning

your configuration file uses a deprecated API spec: "kubeadm.k8s.io/v1beta1". Please use 'kubeadm config migrate --old-config old.yaml --new-config new.yaml

ok so we need to support both versions moving forward, to get rid of that warning.

[EppO]

/lgtm