refactor(templates): move the templates out of the gitea subdirectory

The `templates/gitea` subdirectory did not group anything meaningful, since every template of this chart belongs to
Gitea. It only duplicated the chart name in every path and forced the unit tests to spell out
`templates/gitea/<name>.yaml`, while `_helpers.tpl` and `NOTES.txt` already lived directly in `templates`.

All templates now live in `templates`, which matches the layout of the bundled sub-charts and the Helm defaults.

The checksum helper in `templates/_secrets.tpl` built its include path from `$root.Template.BasePath` and therefore
carried the subdirectory in a `printf` format string instead of a literal path. Without adjusting it the chart failed to
render with "no template gitea/templates/gitea/secret_config.yaml associated with template gotpl".

Co-authored-by: Copilot <copilot@github.com>

unittests/helm/deployment/admin.yaml

@@ -3,16 +3,16 @@ release:
   name: gitea-unittests
   namespace: testing
 templates:
-  - templates/gitea/deployment.yaml
-  - templates/gitea/secret_admin.yaml
-  - templates/gitea/secret_config.yaml
-  - templates/gitea/secret_gpg.yaml
-  - templates/gitea/secret_init.yaml
-  - templates/gitea/secret_inlineConfig.yaml
-  - templates/gitea/secret_metrics.yaml
+  - templates/deployment.yaml
+  - templates/secret_admin.yaml
+  - templates/secret_config.yaml
+  - templates/secret_gpg.yaml
+  - templates/secret_init.yaml
+  - templates/secret_inlineConfig.yaml
+  - templates/secret_metrics.yaml
 tests:
   - it: reads the admin credentials from the generated secret
-    template: templates/gitea/deployment.yaml
+    template: templates/deployment.yaml
     asserts:
       - contains:
           path: spec.template.spec.initContainers[2].env
@@ -45,7 +45,7 @@ tests:
             value: keepUpdated
 
   - it: reads the admin credentials from the configured keys of an existing secret
-    template: templates/gitea/deployment.yaml
+    template: templates/deployment.yaml
     set:
       secrets.admin.existingSecret.enabled: true
       secrets.admin.existingSecret.secretName: custom-admin-secret
@@ -79,7 +79,7 @@ tests:
                 name: custom-admin-secret
 
   - it: omits the admin environment when the admin user is disabled
-    template: templates/gitea/deployment.yaml
+    template: templates/deployment.yaml
     set:
       secrets.admin.enabled: false
     asserts:
@@ -95,7 +95,7 @@ tests:
           any: true
 
   - it: fails on an unsupported password mode
-    template: templates/gitea/deployment.yaml
+    template: templates/deployment.yaml
     set:
       secrets.admin.passwordMode: unsupported
     asserts: