Add dpop to pull_request_target branches allowlist

Temporary entry so PRs in the multi-PR DPoP rollout (targeting the dpop
integration branch) trigger the Pull Request CI workflow. GitHub evaluates
pull_request_target against the default branch (dev) copy of the workflow,
so the dpop branch's copy alone is not sufficient.

Remove this entry once DPoP merges back to dev.

Author: wmathurin

.github/workflows/pr.yaml

@@ -5,7 +5,9 @@ on:
   # Mitigated by per-job Member Check (see "Check Write Permission" + "Validate Write Permission" steps).
   # Reference: team Github Actions Tribal Knowledge doc.
   pull_request_target:  # zizmor: ignore[dangerous-triggers]
-    branches: [dev, master]
+    # dpop is a temporary entry: PRs in the multi-PR DPoP rollout target this
+    # branch. Remove once DPoP is merged back to dev.
+    branches: [dev, master, dpop]
     paths-ignore:
       - '**/*.md'
       - 'LICENSE'