ci: stop testing k8s 1.13

[mboersma]

Reason for Change:
Kubernetes 1.13 has been off the support matrix for a while, so in advance of disabling it, let's stop using CI resources to test it.

AKS is dropping support for 1.13 on Feb. 28th.

Issue Fixed:

Requirements:

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

Notes:

[jackfrancis]

/lgtm

[jackfrancis]

@mboersma actually I take that back, could you also remove 1.13 from the Jenkinsfile at the project root?

[jackfrancis]

/hold

[codecov]

Codecov Report

Merging #2749 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2749   +/-   ##
=======================================
  Coverage   72.34%   72.34%           
=======================================
  Files         137      137           
  Lines       25344    25344           
=======================================
  Hits        18334    18334           
  Misses       5953     5953           
  Partials     1057     1057

[jackfrancis]

/hold release

[jackfrancis]

/lgtm

[acs-bot]

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