chore: deprecate orchestratorType

[jackfrancis]

Reason for Change:

With #4036 merging, we no longer nominally support any non-Kubernetes orchestrators in any code flows (🎉). This change means that the orchestratorType property has no semantic value.

This PR effectively "deprecates" the orchestratorType property in the orchestratorProfile configuration object:

• defaults to Kubernetes where appropriate, so that it no longer needs to be included in API model definitions
• removes several validations whose purpose was to ensure that "orchestratorType == Kubernetes"; because that is always true, those validations are superfluous
• updated documentation and example API models

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #4038 (a85cbba) into master (f5bdb8b) will increase coverage by 0.01%.
The diff coverage is 72.11%.

@@            Coverage Diff             @@
##           master    #4038      +/-   ##
==========================================
+ Coverage   73.06%   73.08%   +0.01%     
==========================================
  Files         145      145              
  Lines       21856    21776      -80     
==========================================
- Hits        15969    15914      -55     
+ Misses       4849     4829      -20     
+ Partials     1038     1033       -5     

Impacted Files	Coverage Δ
cmd/addpool.go	18.18% <0.00%> (+0.10%)	⬆️
cmd/scale.go	12.70% <0.00%> (+0.06%)	⬆️
pkg/api/common/versions.go	96.37% <0.00%> (-1.01%)	⬇️
pkg/api/convertertoagentpoolonlyapi.go	89.47% <ø> (+0.93%)	⬆️
pkg/engine/template_generator.go	67.44% <0.00%> (-0.16%)	⬇️
pkg/engine/templates_generated.go	43.70% <ø> (ø)
pkg/engine/output.go	35.38% <22.50%> (-0.98%)	⬇️
pkg/api/vlabs/validate.go	81.60% <78.41%> (+0.37%)	⬆️
pkg/engine/params_k8s.go	85.13% <84.93%> (-0.10%)	⬇️
pkg/api/apiloader.go	61.41% <100.00%> (+0.42%)	⬆️
... and 9 more

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[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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