Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/linux.yml

@@ -31,13 +31,13 @@ jobs:
             cxx: 'g++'
     steps:
     - name: Fetching QR-Code-generator sources
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
       with:
         repository: 'nayuki/QR-Code-generator'
         ref: 'v1.8.0'
         fetch-depth: 1
     - name: Fetching qrcodegen-cmake sources
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
       with:
         path: 'qrcodegen-cmake'
     - name: Copying CMake build files