Update version of upload-artifact and dowload-artifact to v4

The `v3` of the `actions/upload-artifact` and `actions/download-artifact` actions
got
[deprecated](https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/)
causing possible failure of workflow execution. We're upgrading to `v4` of
the actions. Although there are some breaking changes in `v4` compared to `v3`,
they do not affect our setup.

Author: michalinacienciala

.github/workflows/contracts.yml

@@ -216,7 +216,7 @@ jobs:
           version: ${{ steps.npm-version-bump.outputs.version }}
 
       - name: Upload files needed for etherscan verification
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifa4
         with:
           name: Artifacts for etherscan verification
           path: |
@@ -234,7 +234,7 @@ jobs:
       - uses: actions/checkout@v3
 
       - name: Download files needed for etherscan verification
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: Artifacts for etherscan verification
           path: ./solidity