Enable prevent-mgmt-cluster policy only if kyverno is enabled

Francois Eleouetrequested to merge
fix-capd-kyverno into main

What does this MR do and why?

Fixes errors in capd CI where kyverno is not enabled:

IDENTIFIER                                  STATUS     REASON        MESSAGE
HelmRelease/sylva-units-preview/sylva-units InProgress               HelmRelease generation is 1, but latest observed generation is -1
╰┄╴┬┄┄[Conditions]
   ├┄╴Reconciling                           True       Progressing   Running 'install' action with timeout of 5m0s
   ├┄╴Ready                                 False      InstallFailed Helm install failed for release sylva-units-preview/sylva-units with chart sylva-units@0.0.0-git+f00ae566baf2.1: execution error at (sylva-units/templates/units.yaml:238:18): unit 'kyverno-policy-prevent-mgmt-cluster-delete' is declared with a dependency on disabled unit 'kyverno'
   ╰┄╴Released                              False      InstallFailed Helm install failed for release sylva-units-preview/sylva-units with chart sylva-units@0.0.0-git+f00ae566baf2.1: execution error at (sylva-units/templates/units.yaml:238:18): unit 'kyverno-policy-prevent-mgmt-cluster-delete' is declared with a dependency on disabled unit 'kyverno'

Related reference(s)

Follows-up: !2046 (merged)

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