build(deps): bump actions/upload-artifact from 1 to 2.3.1

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 1 to 2.3.1.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v1...v2.3.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build-docs.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Build documentation
         run: make -C docs html
       - name: Archive build output
-        uses: actions/upload-artifact@v1
+        uses: actions/upload-artifact@v2.3.1
         with:
           name: docs_output
           path: docs/_build/html

.github/workflows/build-gluon.yml

@@ -45,13 +45,13 @@ jobs:
 
       - name: Archive build logs
         if: ${{ !cancelled() }}
-        uses: actions/upload-artifact@v1
+        uses: actions/upload-artifact@v2.3.1
         with:
           name: ${{ matrix.target }}_logs
           path: openwrt/logs
 
       - name: Archive build output
-        uses: actions/upload-artifact@v1
+        uses: actions/upload-artifact@v2.3.1
         with:
           name: ${{ matrix.target }}_output
           path: output