Bump the actions group across 1 directory with 2 updates

Bumps the actions group with 2 updates in the / directory: [actions/checkout](https://github.com/actions/checkout) and [actions/upload-artifact](https://github.com/actions/upload-artifact).


Updates `actions/checkout` from 5 to 6
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

Updates `actions/upload-artifact` from 5 to 6
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: actions
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: actions
...

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

Author: dependabot[bot]

.github/workflows/main.yaml

@@ -8,7 +8,7 @@ jobs:
 
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - uses: actions/setup-python@v6
@@ -19,7 +19,7 @@ jobs:
       contents: write
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - uses: actions/setup-python@v6
@@ -61,15 +61,15 @@ jobs:
       - name: Run checks
         run: |
           make check
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         if: ${{ always() }}
         with:
           name: build
           path: |
             cheatsheets.pdf
             handout-*.pdf
             ./docs/_build/html/
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         id: diffs-artifact-upload
         if: ${{ always() }}
         with: