Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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>

Author: dependabot[bot]

.github/workflows/lint.yaml

@@ -17,7 +17,7 @@ jobs:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
       RENV_PATHS_ROOT: ~/.local/share/renv/
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: r-lib/actions/setup-r@v2
         with:

.github/workflows/pr-comment.yaml

@@ -82,7 +82,7 @@ jobs:
       contents: write
     steps:
       - name: 'Checkout md outputs'
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: md-outputs
           path: built

.github/workflows/pr-receive.yaml

@@ -58,10 +58,10 @@ jobs:
       MD: ${{ github.workspace }}/site/built
     steps:
       - name: "Check Out Main Branch"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: "Check Out Staging Branch"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: md-outputs
           path: ${{ env.MD }}

.github/workflows/sandpaper-main.yaml

@@ -35,7 +35,7 @@ jobs:
     steps:
 
       - name: "Checkout Lesson"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: "Set up R"
         uses: r-lib/actions/setup-r@v2

.github/workflows/update-cache.yaml

@@ -43,7 +43,7 @@ jobs:
       needed: ${{ steps.renv.outputs.exists }}
     steps:
       - name: "Checkout Lesson"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - id: renv
         run: |
           if [[ -d renv ]]; then
@@ -76,7 +76,7 @@ jobs:
     steps:
 
       - name: "Checkout Lesson"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: "Set up R"
         uses: r-lib/actions/setup-r@v2

.github/workflows/update-workflows.yaml

@@ -36,7 +36,7 @@ jobs:
     if: ${{ needs.check_token.outputs.workflow == 'true' }}
     steps:
       - name: "Checkout Repository"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Update Workflows
         id: update