ci(workflows): specify pull_request_target branches (#28328)

Author: caugner

.github/workflows/pr-rebase-needed.yml

@@ -2,6 +2,8 @@ name: PR Needs Rebase
 on:
   push:
   pull_request_target:
+    branches:
+      - main
     types: [synchronize]
 
 permissions:

.github/workflows/system-file-changes.yml

@@ -2,6 +2,8 @@ name: System file changes
 
 on:
   pull_request_target:
+    branches:
+      - main
     paths:
       - "**"
       - "!**.md"

.github/workflows/update-mdn-urls.yml

@@ -2,6 +2,8 @@ name: Update MDN urls
 
 on:
   pull_request_target:
+    branches:
+      - main
     paths:
       - "package-lock.json"