Drop use of comprimised action

https://www.stepsecurity.io/blog/harden-runner-detection-tj-actions-changed-files-action-is-compromised

Author: tstellar

.github/workflows/docs.yml

@@ -65,7 +65,7 @@ jobs:
           fetch-depth: 1
       - name: Get subprojects that have doc changes
         id: docs-changed-subprojects
-        uses: tj-actions/changed-files@v39
+        uses: step-security/changed-files@3dbe17c78367e7d60f00d78ae6781a35be47b4a1
         with:
           files_yaml: |
             llvm: