Bump actions/checkout from 2 to 6 (#28)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...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] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/CI.yml

@@ -25,7 +25,7 @@ jobs:
         arch:
           - x64
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v1
         with:
           version: ${{ matrix.version }}
@@ -41,7 +41,7 @@ jobs:
     name: Documentation
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v1
         with:
           version: '1'

.github/workflows/downgrade.yml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         version: ['1.10']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v1
         with:
           version: ${{ matrix.version }}