Bump actions/checkout from 2 to 4 (#21)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  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>

.github/workflows/CI.yml

@@ -33,7 +33,7 @@ jobs:
             arch: x64
             downgrade: true
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v1
         with:
           version: ${{ matrix.version }}
@@ -46,7 +46,7 @@ jobs:
         name: Downgrade dependencies to oldest supported versions
       - uses: julia-actions/julia-buildpkg@v1
       - name: Clone posteriordb
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           repository: stan-dev/posteriordb
           path: 'posteriordb'
@@ -66,7 +66,7 @@ jobs:
     permissions:
       contents: write
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v1
         with:
           version: '1'

.github/workflows/UpdateArtifacts.yml

@@ -13,7 +13,7 @@ jobs:
           - artifact_name: posteriordb
             github_repo: stan-dev/posteriordb
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Get latest artifact release
         id: latestrelease
         uses: pozetroninc/github-action-get-latest-release@master