Bump actions/download-artifact from 7 to 8 (#713)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 7 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v7...v8)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '8'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -310,39 +310,39 @@ jobs:
       - uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: ubuntu-latest-3.9
           path: /tmp/o39
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: ubuntu-latest-3.10
           path: /tmp/o310
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: ubuntu-latest-3.11
           path: /tmp/o311
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: ubuntu-latest-3.12
           path: /tmp/o312
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: ubuntu-latest-3.13
           path: /tmp/o313
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: macos-latest-3.9
           path: /tmp/m39
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: macos-latest-3.11
           path: /tmp/m311
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: windows-latest-3.9
           path: /tmp/w39
-      - uses: actions/download-artifact@v7
+      - uses: actions/download-artifact@v8
         with:
           name: windows-latest-3.12
           path: /tmp/w312