[dendrite] Upgrade app version and change default value to Github Containers

Description of the change

Upgraded app version to 0.9.3 and changed to use ghcr.io.

Benefits

Will not hit Docker Hub rate limits and newer software.

Additional information

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.

k8s-at-home / charts