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/ci.yml

@@ -73,7 +73,7 @@ jobs:
       NIM_ARCH: ${{ matrix.target.cpu }}
     steps:
       - name: Checkout threading
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: threading
           submodules: false

.github/workflows/doc.yml

@@ -13,7 +13,7 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: jiro4989/setup-nim-action@v2
         with:
           nim-version: 'devel'