CI: Update actions/checkout to v6

Node.js 20 actions are deprecated. The following actions are running on Node.js
20 and may not work as expected: actions/checkout@v4. Actions will be forced to
run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be
removed from the runner on September 16th, 2026. Please check if updated
versions of these actions are available that support Node.js 24. To opt into
Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment
variable on the runner or in your workflow file. Once Node.js 24 becomes the
default, you can temporarily opt out by setting
ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see:
https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Author: andre-schulz

.github/workflows/main.yml

@@ -42,7 +42,7 @@ jobs:
       QMAKE_SPEC: ${{ matrix.platform.qmake_spec }}
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Install Ubuntu dependencies
       if: runner.os == 'Linux'