Bump actions/checkout from 3 to 6 (#18)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  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>

Author: dependabot[bot]

.github/workflows/CI.yml

@@ -16,7 +16,7 @@ jobs:
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - id: set-matrix
         run: |
           set -e
@@ -40,7 +40,7 @@ jobs:
         with:
           cache-name: julia-cache;workflow=${{ github.workflow }};ref=${{ github.run_id }}
           include-matrix: false
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: "Install Jupyter"
         run: |
           if (( $(echo "$(pip --version | awk '{print $2}' | cut -d'.' -f 1,2) >= 23.1" |bc -l) )); then

.github/workflows/keep-repo-alive.yml

@@ -9,7 +9,7 @@ jobs:
   whitespace:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: Append to whitespace database
         run: echo -n " " >> whitespace.txt
       - name: Commit report