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] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -11,7 +11,7 @@ jobs:
   ormolu:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: haskell-actions/run-ormolu@v17
   build:
     runs-on: ubuntu-latest
@@ -21,7 +21,7 @@ jobs:
         cabal: ["3.12"]
         ghc: ["9.8.4", "9.10.3", "9.12.1"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: haskell-actions/setup@v2.8.2
         id: setup-haskell-cabal
         with: