two-step Sylva upgrade: Flux first and then the rest

During a Sylva upgrade to a new minor version (Y in x.Y.z), for instance Sylva 1.4 to 1.5, to use new Flux features, we need the ability to upgrade Flux first to a the version of Flux introducing the new features, and then upgrade sylva-units Helm chart to make use of them.

(We can't have sylva-units Helm release try to use them before we have upgraded Flux -- even when a new feature is introduced without a change of apiVersion, we run into issues like #2375 (closed) )

One solution that has been discussed would be to evolve SylvaUnitsRelease operator -- on an upgrade of sylva-units it would:

• create a new source for sylva-units/sylva-core (e.g. GitRepo)
• first update sylva-units HelmRelease to override values to
  • define a new entry under source_templates pointing to the new sylva-core source sylva-core-next
  • override units.flux-system.repo to point to sylva-core-next

This assumes that we make use of SylvaUnitsRelease, including for the CLI-based workflow.

/cc @feleouet @vladimir.braquet