Bump actions/checkout from 2 to 4 (#27)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.

---

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
     name: Documentation
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: r-lib/actions/setup-r@v2
       - run: echo "LD_LIBRARY_PATH=$(R RHOME)/lib:$LD_LIBRARY_PATH" >> $GITHUB_ENV
       - uses: julia-actions/setup-julia@v2