Re-add GPG configuration feature (#374)

This reverts d5ce1a47ea and therefore adds the GPG feature back into main.
As it is a breaking change, this PR now also contains the required upgrade notes.

Closes #107 again.

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/374
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>

unittests/gpg-secret/signing-disabled.yaml

@@ -0,0 +1,13 @@
+suite: GPG secret template (signing disabled)
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/gpg-secret.yaml
+tests:
+  - it: renders nothing
+    set:
+      signing.enabled: false
+    asserts:
+      - hasDocuments:
+          count: 0