feat(gh-templates): Require non-duplicate confirmation on FR (#8618)

.github/DISCUSSION_TEMPLATE/feature-request.yaml

@@ -6,6 +6,13 @@ body:
     attributes:
       value: |
         Please use this form to request new feature for Immich
+  
+  - type: checkboxes
+    attributes:
+      options:
+        - label: I have searched the existing feature requests to make sure this is not a duplicate request.
+          required: true
+  
   - type: textarea
     id: feature
     attributes: