Remove all Qt5 < 5.15 workarounds.

Qt 5.15 is now the minimal required Qt version.

.github/workflows/linux.yml

@@ -8,7 +8,10 @@ on:
 jobs:
   build:
     name: QtPBFImagePlugin
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        config: ['release', 'debug']
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -17,6 +20,6 @@ jobs:
           sudo apt-get update
           sudo apt-get install qtbase5-dev qtbase5-dev-tools qt5-qmake zlib1g-dev
       - name: Configure build
-        run: qmake pbfplugin.pro
+        run: qmake CONFIG+=${{ matrix.config }} pbfplugin.pro
       - name: Build project
         run: make -j2