ci: update GitHub Actions to use latest artifact actions

- Upgraded actions/download-artifact from v5 to v7.
- Upgraded actions/upload-artifact from v3 to v6.

Author: ekalinin

.github/workflows/main.yml

@@ -63,7 +63,7 @@ jobs:
         run: python -m pip install --upgrade coverage[toml]
 
       - name: Download data
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v7
         with:
           name: coverage-data
 
@@ -75,7 +75,7 @@ jobs:
 
       - name: Upload HTML report
         if: ${{ failure() }}
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v6
         with:
           name: html-report
           path: htmlcov