ci: upgrade upload-artifact and download-artifact to v4

Apparently v2 is deprecated and no longer supported. v3 works for us and
v4 has some new limit on the size of uploads, but those shouldn't be a
problem here, so jump all the way to v4.

Author: apoelstra

.github/workflows/fuzz.yml

@@ -28,7 +28,7 @@ jobs:
       - name: fuzz
         run: cd fuzz && ./fuzz.sh "${{ matrix.fuzz_target }}"
       - run: echo "${{ matrix.fuzz_target }}.rs" >executed_${{ matrix.fuzz_target }}
-      - uses: actions/upload-artifact@v2
+      - uses: actions/upload-artifact@v4
         with:
           name: executed_${{ matrix.fuzz_target }}
           path: executed_${{ matrix.fuzz_target }}
@@ -39,7 +39,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/download-artifact@v2
+      - uses: actions/download-artifact@v4
       - name: Display structure of downloaded files
         run: ls -R
       - run: find executed_* -type f -exec cat {} + | sort > executed