Bump actions/checkout from 3 to 6

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>

Author: dependabot[bot]

.github/workflows/check-standalone.yaml

@@ -26,7 +26,7 @@ jobs:
       R_KEEP_PKG_SOURCE: yes
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       - uses: r-lib/actions/setup-pandoc@v2
 
@@ -74,7 +74,7 @@ jobs:
         shell: Rscript {0}
 
       - name: Checkout RStan Experimental branch
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
         with:
           repository: stan-dev/rstan
           ref: experimental

.github/workflows/check-standard.yaml

@@ -30,7 +30,7 @@ jobs:
       R_KEEP_PKG_SOURCE: yes
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       - uses: r-lib/actions/setup-pandoc@v2