build.yaml: Replace dead ubuntu-18.04 by current ubuntu-24.04

Author: hartwork

.github/workflows/build.yaml

@@ -26,10 +26,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - runs-on: ubuntu-24.04
+            qt: qt5-qmake
           - runs-on: ubuntu-22.04
             qt: qt5-qmake
-          - runs-on: ubuntu-18.04
-            qt: qt5-default
 
     name: Build (Linux, ${{ matrix.runs-on }})
     runs-on: ${{ matrix.runs-on }}