Deprecate lookup-path option and updated relevant documentation

[rhuss]

Lookup in path is now the default for all plugins.
This option will be removed eventually in a future version,
when path lookup is enabled uncoditinally and can't be turned of.

[codecov]

Codecov Report

Merging #1422 (4c11afe) into main (f8a8284) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1422      +/-   ##
==========================================
- Coverage   78.68%   78.68%   -0.01%     
==========================================
  Files         162      162              
  Lines        8361     8359       -2     
==========================================
- Hits         6579     6577       -2     
  Misses       1098     1098              
  Partials      684      684              

Impacted Files	Coverage Δ
pkg/kn/commands/plugin/list.go	83.33% <100.00%> (ø)
pkg/kn/config/config.go	60.60% <100.00%> (-0.79%)	⬇️

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[dsimansk]

@rhuss and you'll probably have conflict in changelog once --tls PR #1419 is merged.

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/config/config.go	75.3%	74.7%	-0.5

[dsimansk]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, rhuss

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