Adds an app_version input that overrides the chart's appVersion
via helm package --app-version. When unset, the flag is omitted
and the appVersion from Chart.yaml is preserved, keeping existing
behavior unchanged.

feat(helm-publish): add optional app_version input

See merge request !1553

## [4.7.0](https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/compare/...) (2026-06-02)

### Features

* **helm-publish:** add optional app_version input ([8c439e79](8c439e79))