fix: make pr_number input optional in dual-approval-check

PR #81 adds pr_number as a required input, but the 5 calling
plugin repos don't yet pass this input. Change required: true
to required: false so the workflow_call can succeed without it,
while companion PRs are opened to add the input to each caller.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/dual-approval-check.yaml

@@ -22,7 +22,7 @@ on:
     inputs:
       pr_number:
         description: "Pull request number"
-        required: true
+        required: false
         type: number
       cto-reviewer:
         description: "GitHub username of the CTO reviewer"