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.yml

@@ -17,7 +17,7 @@ jobs:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - uses: dtolnay/rust-toolchain@nightly
@@ -34,7 +34,7 @@ jobs:
     name: Format
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: mkroening/rust-toolchain-toml@main
       - run: rustup component add rustfmt
       - name: Format
@@ -48,7 +48,7 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y qemu-kvm
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
          submodules: true
     - uses: dtolnay/rust-toolchain@stable