Helm v4 support

[jnoordsij]

With Helm v4 having its first rc (see https://github.com/helm/helm/releases/tag/v4.0.0-rc.1) I decided to check for plugin support, finding this plugin to be not entirely compatible yet. This is a rough attempt at updating to be compatible with Helm v4. I'm unsure about the full implications of support for Helm v3 by changes introduced in this PR. Any suggestions or help are welcome.

For reference, the only issue I found with running this with Helm v4 was the deprecation/error:

Error: Failed to render chart: exit status 1: Flag --validate has been deprecated, use '--dry-run=server' instead
Error: if any flags in the group [validate dry-run] are set none of the others can be; [dry-run validate] were all set

One can already circumvent this with export HELM_DIFF_USE_UPGRADE_DRY_RUN=true; making this the default (as proposed by this PR) seems the way forward, but might have other technical implications.

[yxxhero]

@jnoordsij

Error: Failed to render chart: exit status 1: Flag --validate has been deprecated, use '--dry-run=server' instead
Error: if any flags in the group [validate dry-run] are set none of the others can be; [dry-run validate] were all set

we should use '--dry-run=server' when use helm v4, so we should check the helm verision and choice the right option to apply.