Add support for Helm v4

[stefanprodan]

Helm v4 had it's first alpha release so we should investigate what changes are needed to helm-controller and source-controller to migrate them to the Helm v4 SDK.

Once Helm v4 makes a GA release, we should be prepare to release helm-controller & source-controller with v4 support. I've added the migration to the roadmap for Flux 2.8 in case v4 goes GA by end of 2025 and we hit no roadblocks with the SDK.

⚠️ If there are any breaking changes to Helm charts produced with Helm v3, we will not offer any backwards compatibility beyond what the Helm SDK offers. Flux users will have to pin helm-controller to v1.4.0 until they migrate their charts to v4. Given that source-controller builds charts from Git, if the Helm SDK v4 can not work with v3 chart sources, then Flux users will have to pin source-controller to v1.7.0.

[hoerup]

Is there coming anything in helm4 that will help resolve #896 or is completely unrelated ?

[danieldides]

Helm v4 has released helm/helm#31488

[matheuscscp]

We are already working towards Helm 4 support in Flux 2.8, scheduled for release on Feb 21st, one month after Helm 4.1 is released.