chore: remove support for Kubernetes 1.15

[mboersma]

Reason for Change:

This PR removes support for creating new clusters with Kubernetes 1.15.

Issue Fixed:

Fixes #3631

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[mboersma]

Hmm...

[jackfrancis]

Will delete this test config entirely

[jadarsie]

A bunch of these {{- if IsKubernetesVersionGe "1.16.0"}} can be removed from the templates under parts/k8s.

Example: https://github.com/Azure/aks-engine/blob/master/parts/k8s/addons/kube-proxy.yaml#L51-L54

[mboersma]

{{- if IsKubernetesVersionGe "1.16.0"}} can be removed from the templates

You are so right! Thanks, I'll make those changes and update this PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0b6b43b). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #3751   +/-   ##
=========================================
  Coverage          ?   72.79%           
=========================================
  Files             ?      149           
  Lines             ?    23158           
  Branches          ?        0           
=========================================
  Hits              ?    16858           
  Misses            ?     5183           
  Partials          ?     1117           

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/engine/template_generator.go	81.31% <ø> (ø)
pkg/engine/templates_generated.go	48.97% <ø> (ø)
pkg/api/addons.go	97.75% <100.00%> (ø)
pkg/engine/artifacts.go	98.95% <0.00%> (ø)
pkg/api/agentPoolOnlyApi/v20170831/merge.go	0.00% <0.00%> (ø)
pkg/armhelpers/azurestack/compute.go	31.57% <0.00%> (ø)
pkg/engine/networksecuritygroups.go	100.00% <0.00%> (ø)
pkg/engine/userassignedidentities.go	100.00% <0.00%> (ø)
cmd/generate.go	49.18% <0.00%> (ø)
... and 143 more

---

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

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[acs-bot]

New changes are detected. LGTM label has been removed.