Add deprecation fail-safe for Chart templating (#269)

With release 5.0.0 there are so many deprecations and breaking changes
that it is probably a good way to assist the users with values migration
before breaking their environments.

This adds another template file that doesn't render anything but ensures
the removal of dropped or deprecated settings from customized values
files.

For when it is necessary, this check can be disabled via new setting
`checkDeprecation`.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/269
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: wxiaoguang <wxiaoguang@noreply.gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

.drone.yml

@@ -15,6 +15,14 @@ steps:
   - apk add --no-cache -X http://dl-cdn.alpinelinux.org/alpine/edge/testing helm
   - helm lint
 
+- name: helm template
+  pull: always
+  image: alpine:3.13
+  commands:
+  - apk add --no-cache -X http://dl-cdn.alpinelinux.org/alpine/edge/testing helm
+  - helm dependency update
+  - helm template --debug gitea-helm .
+
 - name: markdown lint
   pull: always
   image: docker.io/volkerraschek/markdownlint:latest