[gotify] Upgraded to app version 2.1.5

Description of the change

Upgraded to gotify server 2.1.5 Switched from using Docker hub to GitHub image repository

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.
[x] Variables have been documented in the values.yaml file.

k8s-at-home / charts