chore(deps): update all dependencies to v4 (#12133)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[actions/download-artifact](https://togithub.com/actions/download-artifact)
| action | major | `v3` -> `v4` |
|
[actions/upload-artifact](https://togithub.com/actions/upload-artifact)
| action | major | `v3` -> `v4` |

---

### Release Notes

<details>
<summary>actions/download-artifact (actions/download-artifact)</summary>

###
[`v4`](https://togithub.com/actions/download-artifact/compare/v3...v4)

[Compare
Source](https://togithub.com/actions/download-artifact/compare/v3...v4)

</details>

<details>
<summary>actions/upload-artifact (actions/upload-artifact)</summary>

### [`v4`](https://togithub.com/actions/upload-artifact/compare/v3...v4)

[Compare
Source](https://togithub.com/actions/upload-artifact/compare/v3...v4)

</details>

---

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Co-authored-by: Anthonios Partheniou <partheniou@google.com>

.github/workflows/unittest.yml

@@ -38,9 +38,9 @@ jobs:
       run: |
         ci/run_conditional_tests.sh
     - name: Upload coverage results
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
-        name: coverage-artifacts
+        name: coverage-artifact-${{ '{{' }} matrix.python {{ '}}' }}
         path: .coverage-${{ matrix.python }}
   prerelease:
     runs-on: ubuntu-latest
@@ -97,12 +97,12 @@ jobs:
         python -m pip install coverage
     - name: Download coverage results
       if: ${{ steps.date.packages.num_files_changed > 0 }}
-      uses: actions/download-artifact@v3
+      uses: actions/download-artifact@v4
       with:
-        name: coverage-artifacts
         path: .coverage-results/
     - name: Report coverage results
       if: ${{ steps.date.packages.num_files_changed > 0 }}
       run: |
-        coverage combine .coverage-results/.coverage*
+        find .coverage-results -type f -name '*.zip' -exec unzip {} \;
+        coverage combine .coverage-results/**/.coverage*
         coverage report --show-missing --fail-under=100