[ghost] Upgrade appVersion to 5.8.3

[samip5]

Fixes a security vulnerability: https://github.com/TryGhost/Ghost/security/advisories/GHSA-7v28-g2pq-ggg8

Checklist

• [x] Title of the PR starts with chart name (e.g. [home-assistant])
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [ ] Variables have been documented in the values.yaml file.