Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/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)

---
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]>

Author: dependabot[bot]

.github/workflows/haskell-ci.yml

@@ -222,7 +222,7 @@ jobs:
           chmod a+x $HOME/.cabal/bin/cabal-plan
           cabal-plan --version
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: source
       - name: initial cabal.project for sdist

.github/workflows/stack.yml

@@ -29,7 +29,7 @@ jobs:
     #   upload_url: ${{ steps.create_release.outputs.upload_url }}
     steps:
 
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
 
     # - name: Create release
     #   id: create_release
@@ -95,7 +95,7 @@ jobs:
         shell: bash
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
 
     - uses: haskell-actions/setup@v2
       id: setup