Fix multi-doc parser of --- for post renderers

[matheuscscp]

Contains: helm/helm#31941

After the fix is released in Helm 4.1.4, we will remove this replace and delete the fluxcd/helm fork.

xref:

• Regression from Helm 3: annotateAndMerge corrupts manifests when rendered template content has --- followed by no newline helm/helm#31867
• Regression for templates containing consecutive groups of 3 dashes after document separator fix helm/helm#31935

[matheuscscp]

I tested this and both bugs are fixed:

● Both pass:

  - repro-glued (the ---\n{{- include ...}} glued separator issue from #1423): Install succeeded
  - victoria-logs (the Grafana dashboard JSON with "---------" regression from #31935): Install
  succeeded

[stefanprodan]

LGTM 🚀

[fluxcdbot]

Successfully created backport PR for release/v1.5.x:

• [release/v1.5.x] Fix multi-doc parser of --- for post renderers #1443