chore: remove support for Kubernetes 1.13

[mboersma]

Reason for Change:
1.13 is unsupported in AKS now and has long been out of community support.

Issue Fixed:

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3059 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3059      +/-   ##
==========================================
- Coverage   70.63%   70.57%   -0.06%     
==========================================
  Files         145      145              
  Lines       25151    25151              
==========================================
- Hits        17765    17751      -14     
- Misses       6283     6299      +16     
+ Partials     1103     1101       -2     

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.55% <ø> (ø)
pkg/engine/templates_generated.go	30.99% <0.00%> (-0.59%)	⬇️

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[jackfrancis]

/lgtm