Pin helm-charts to v5 (RC validation, then GA)

[nscuro]

1. RC validation pin (during the apiserver soak)

Open a PR pinning the default image.tag for both apiserver and frontend to 5.0.0-rc.1. Use this to validate the chart
end-to-end against the RC. Do not merge this PR to a chart release. It exists only to exercise the chart against real
v5 images.

2. GA pin (after apiserver GA and frontend GA)

Update charts/dependency-track/values.yaml. Set the default image.tag for apiserver and frontend to 5.0.0. The image names are already correct (dependencytrack/apiserver and dependencytrack/frontend).
No rename needed.

Bump Chart.yaml's appVersion to 5.0.0 and the chart version per the chart's own semver policy. Update the README
and values examples.

Decide and document the :snapshot policy for the development chart. If the chart pins image.tag: snapshot, choose one
of two options. Either pin the v5 dev chart to 5-snapshot (a clean break, recommended), or keep bare snapshot and let
the chart follow v4 nightly until v4 EOL. Document the choice in the chart README.

Last, verify chart distribution URL stability. helm-charts is not in the archive set, so index.yaml and chart
tarball URLs should be unaffected. Confirm:

• The index.yaml URL still resolves.
• Chart tarball entries reference URLs that survive.
• No badge or link in the chart README points at archived hyades-apiserver or hyades-frontend.

The RC validation PR depends on the apiserver RC images being on Docker Hub. The GA pin depends on both apiserver GA and
frontend GA being out.