Cleanups following the introduction of sylva-units-operator in CLI workflow

Summary

Sylva-units operator will be introduced in CLI workflow in !4101 (merged)

• In order to limit code change, this MR was trying to limit the code change, but various parts could be refactored following this introduction:

reconcile_sylva_units function can be simplified, since we don't have to suspend & resume sylva-units any more as the operator and the use of nameGenerators will ensure that there are no more race conditions.

• Once CI won't be testing upgrade from previous releases without this operator, we'll be able to remove that patch and these CI values

• For the CI, we should be able to use valuesFrom instead of merging the values using yq (this probably deserves a dedicated issue)

Additionally, once sylva-projects/sylva-elements/sylva-units-operator#52 (closed) will be implemented:

We'll be able to revisit the way we add extra botostrap files that was introduced in !5330 (merged), in particular we'll be able to remove these parts:

• https://gitlab.com/sylva-projects/sylva-core/-/blob/fa805b46a618f4a3bcb6bf786765e444f6d375d8/charts/sylva-units/scripts/management-cluster-configs.sh#L25
• https://gitlab.com/sylva-projects/sylva-core/-/blob/360b1defb54faacf8ef2b0ec21f4a4d890c28b48/tools/shell-lib/common.sh#L432-449

Similarily, we'll be ble to simplify preview.sh and validate_sylva_units, as we'll just have to enable the preview context to preview management or workload cluster.