Actions(deps): Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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/linux.yml

@@ -75,7 +75,7 @@ jobs:
               clang-${{ matrix.clang_major_version }}
 
       - name: Checkout libprojectM Sources
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: projectM-visualizer/projectm
           path: libprojectM
@@ -95,7 +95,7 @@ jobs:
           cmake --install "${{ github.workspace }}/cmake-build-libprojectM"
 
       - name: Checkout Git branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: libvisual-projectm