feat: disable dashboard addon by default

[mboersma]

Reason for Change:
The kubernetes-dashboard implementation is stale and not considered compatible with current versions of Kubernetes, so this change disables it by default. It can still be turned on if desired by enabling it explicitly in a cluster config (apimodel).

Issue Fixed:
Fixes #3021

Requirements:

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

Notes:
Users can try out the new pre-release versions by following the current documentation for Kubernetes Dashboard.

[codecov]

Codecov Report

Merging #3093 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3093      +/-   ##
==========================================
- Coverage   70.64%   70.61%   -0.03%     
==========================================
  Files         147      147              
  Lines       25430    25430              
==========================================
- Hits        17965    17958       -7     
- Misses       6342     6350       +8     
+ Partials     1123     1122       -1     

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	35.21% <0.00%> (-0.31%)	⬇️

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[jackfrancis]

/lgtm