Drop building of Qt 5.15 (#727)

Author: dail8859

.github/workflows/build.yml

@@ -16,11 +16,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, windows-latest, macos-latest]
-        qt_version: ["5.15", "6.5", "6.6"]
-        exclude:
-        - os: macos-latest
-          qt_version: "5.15"
+        os: [ubuntu-22.04, windows-latest, macos-latest]
+        qt_version: ["6.5"]
 
     runs-on: ${{ matrix.os }}
 
@@ -34,7 +31,7 @@ jobs:
         uses: jurplel/install-qt-action@v4
         with:
           version: ${{ matrix.qt_version }}
-          modules: ${{ matrix.qt_version != '5.15' && 'qt5compat' || '' }}
+          modules: 'qt5compat'
           cache: true
 
       - name: Get number of CPU cores

README.md

@@ -50,7 +50,7 @@ defaults -currentHost write -g AppleFontSmoothing -int 0
 A restart is required for this to take effect.
 
 # Development
-Current development is done using QtCreator with the Microsft Visual C++ (msvc) compiler. Qt 6.5 is the prefered Qt version but this can also be built with Qt 5.15. This is also known to build successfully on various Linux distributions and macOS. Other platforms/compilers should be usable with minor modifications.
+Current development is done using QtCreator with the Microsft Visual C++ (msvc) compiler. Qt 6.5 is the currently supported Qt version. Older versions of Qt are likely to work but are not tested. Any fixes for older versions will be accepted as long as they do not introduce complex fixes. This application is also known to build successfully on various Linux distributions and macOS. Other platforms/compilers should be usable with minor modifications.
 
 If you are familiar with building C++ Qt desktop applications with Qt Creator, then this should be as simple as opening `src/NotepadNext.pro` and build/run the project.