CI: Update actions/checkout to v6

The CI pipelines warn about Actions still using Node.js 20 because
Node.js 20 has reached end-of-life on April 2026 and Actions will be
forced to use Node.js 24 on June 2nd, 2026, see [1].

Update actions/checkout to v6, which uses Node.js 24, to get rid of the
warning.

[1] https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Author: andre-schulz

.github/workflows/main.yml

@@ -37,7 +37,7 @@ jobs:
       QMAKE_SPEC: ${{ matrix.platform.qmake_spec }}
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Install Ubuntu dependencies
       if: runner.os == 'Linux'