github: update action version for build-pdf

- upload-artifact@v4
- download-artifact@v4

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>

.github/workflows/build-pdf.yml

@@ -25,7 +25,7 @@ jobs:
         run: make -C doc build
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
           name: v-intrinsic-spec.pdf
           path: doc/v-intrinsic-spec.pdf
@@ -73,7 +73,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Download artifact
-        uses: actions/download-artifact@v2
+        uses: actions/download-artifact@v4
         with:
           name: v-intrinsic-spec.pdf
           path: ./doc