workflow-diagram: don't allow connections to trigger nodes

[josephjclark]

Validation Steps

• Create an orphaned node by deleting its parent edge
• Try to create a connection from the orphaned node to a trigger node
• The trigger should be greyed out on drag, and show an error on hover

Related issue

Fixes #2188

Review checklist

• [x] I have performed a self-review of my code
• [x] I have verified that all appropriate authorization policies (:owner, :admin, :editor, :viewer) have been implemented and tested
• [x] If needed, I have updated the changelog
• [x] Product has QA'd this feature

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.68%. Comparing base (9423bd7) to head (6dfe96e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2223 +/- ## ========================================== + Coverage 89.67% 89.68% +0.01% ========================================== Files 275 275 Lines 9055 9055 ========================================== + Hits 8120 8121 +1 + Misses 935 934 -1 ```

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