Simplify version handling (#250)

- Drop super legacy `image.version` value (see #92 description)
- Always use `appVersion` from Chart.yaml as image tag if non specified

---

Don't know whether this is a breaking change regarding image.version
drop.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/250
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

values.yaml

@@ -8,7 +8,8 @@ clusterDomain: cluster.local
 
 image:
   repository: gitea/gitea
-  tag: 1.16.2
+  # Overrides the image tag whose default is the chart appVersion.
+  tag: ""
   pullPolicy: Always
   rootless: false # only possible when running 1.14 or later