Make code Qt 6 compatible / raises minimum Qt 5 version to 5.14

In addition to replacing Qt5 with Qt6 in CMakeLists.txt
two small changes to the code are needed to make things build
on Qt 6 (which in turn is needed for Apple M1/M2 support).

The cost of this change is that when building on Qt 5, at least
version 5.14 is needed.
Certain older but still used Linux distributions such as
Ubuntu 10.04 LTS still use 5.12, so revert this commit if you
are building on those...

src/CMakeLists.txt

@@ -100,7 +100,7 @@ set(SOURCES "main.cpp" "imagewriter.cpp" "networkaccessmanagerfactory.cpp"
     "devicewrapper.cpp" "devicewrapperblockcacheentry.cpp" "devicewrapperpartition.cpp" "devicewrapperfatpartition.cpp"
     "driveformatthread.cpp" "localfileextractthread.cpp" "powersaveblocker.cpp" "downloadstatstelemetry.cpp" "qml.qrc" "dependencies/sha256crypt/sha256crypt.c" "cli.cpp")
 
-find_package(Qt5 COMPONENTS Core Quick LinguistTools Svg OPTIONAL_COMPONENTS Widgets)
+find_package(Qt5 5.14 COMPONENTS Core Quick LinguistTools Svg OPTIONAL_COMPONENTS Widgets)
 if (Qt5Widgets_FOUND)
     set(EXTRALIBS ${EXTRALIBS} Qt5::Widgets)
 endif()

src/OptionsPopup.qml

@@ -3,7 +3,7 @@
  * Copyright (C) 2021 Raspberry Pi Ltd
  */
 
-import QtQuick 2.9
+import QtQuick 2.15
 import QtQuick.Controls 2.2
 import QtQuick.Layouts 1.0
 import QtQuick.Controls.Material 2.2
@@ -128,9 +128,7 @@ Popup {
                             id: fieldHostname
                             enabled: chkHostname.checked
                             text: "raspberrypi"
-                            /* FIXME: use RegularExpressionValidator instead when moving to newer Qt version.
-                               It is not available in 5.12 that is still being used by Ubuntu 20 LTS though */
-                            validator: RegExpValidator { regExp: /[0-9A-Za-z][0-9A-Za-z-]{0,62}/ }
+                            validator: RegularExpressionValidator { regularExpression: /[0-9A-Za-z][0-9A-Za-z-]{0,62}/ }
                         }
                         Text {
                             text : ".local"

src/devicewrapperfatpartition.cpp

@@ -1,6 +1,5 @@
 #include "devicewrapperfatpartition.h"
 #include "devicewrapperstructs.h"
-#include "qdebug.h"
 
 /*
  * SPDX-License-Identifier: Apache-2.0
@@ -314,7 +313,7 @@ void DeviceWrapperFatPartition::writeFile(const QString &filename, const QByteAr
     for (uint32_t cluster : qAsConst(clusterList))
     {
         seekCluster(cluster);
-        write(contents.data()+pos, qMin(_bytesPerCluster, contents.length()-pos));
+        write(contents.data()+pos, qMin((qsizetype)_bytesPerCluster, (qsizetype)(contents.length()-pos)));
 
         pos += _bytesPerCluster;
         if (pos >= contents.length())