Update .appveyor.yml from QT v6.1 --> v6.5

Looks like AppVeyor dropped support for v6.1 QT as shown in https://www.appveyor.com/docs/windows-images-software/

Update to latest QT version

.appveyor.yml

@@ -8,7 +8,7 @@ install:
 - cmd: >-
     if /i "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2017" (call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" x86) & (set QTDIR=C:\Qt\5.13\msvc2017)
 
-    if /i "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2019" (call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64) & (set QTDIR=C:\Qt\6.1\msvc2019_64)
+    if /i "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2019" (call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64) & (set QTDIR=C:\Qt\6.5\msvc2019_64)
 
     set path=%PATH%;%QTDIR%\bin
 build_script: