Update msvc build for Visual Studio 2019 v16.4

msvc warning C4834 for the Bitcoin Core build was introduced by Visual Studio 16.4.0. This PR adds an ignore rule for the warning (it's related to the nodiscard attribute and is not considered relevant).
An additional side effect of the msvc compiler update is the prebuilt Qt5.9.8 libraries cannot be linked due to being built with an earlier version of the compiler. To fix this a new Qt5.9.8 version has been compiled and the appveyor job updated to use them. The GitHub Actions job needs to continue to use the original Qt5.9.8 libraries until the latest GitHub Windows image also updates to >= Visual Studio 2019 v16.4.

Github-Pull: bitcoin#17736
Rebased-From: 75d9317

.appveyor.yml

@@ -9,8 +9,8 @@ environment:
   CLCACHE_SERVER: 1
   PATH: 'C:\Python37-x64;C:\Python37-x64\Scripts;%PATH%'
   PYTHONUTF8: 1
-  QT_DOWNLOAD_URL: 'https://github.com/sipsorcery/qt_win_binary/releases/download/v1.4/Qt5.9.8_x64_static_vs2019.zip'
-  QT_DOWNLOAD_HASH: 'f285cbb02bec3b3f3cc2621e3fa7d5edf0d6a66fa30c57859e583acda954ea80'
+  QT_DOWNLOAD_URL: 'https://github.com/sipsorcery/qt_win_binary/releases/download/v1.6/Qt5.9.8_x64_static_vs2019.zip'
+  QT_DOWNLOAD_HASH: '9a8c6eb20967873785057fdcd329a657c7f922b0af08c5fde105cc597dd37e21'
   QT_LOCAL_PATH: 'C:\Qt5.9.8_x64_static_vs2019'
   VCPKG_INSTALL_PATH: 'C:\tools\vcpkg\installed'
 cache:

build_msvc/common.init.vcxproj

@@ -107,7 +107,7 @@
       <WarningLevel>Level3</WarningLevel>
       <PrecompiledHeader>NotUsing</PrecompiledHeader>
       <AdditionalOptions>/utf-8 %(AdditionalOptions)</AdditionalOptions>
-      <DisableSpecificWarnings>4018;4221;4244;4267;4334;4715;4805;</DisableSpecificWarnings>
+      <DisableSpecificWarnings>4018;4221;4244;4267;4334;4715;4805;4834</DisableSpecificWarnings>
       <TreatWarningAsError>true</TreatWarningAsError>
       <PreprocessorDefinitions>ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_SCL_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>..\..\src;..\..\src\univalue\include;..\..\src\secp256k1\include;..\..\src\leveldb\include;..\..\src\leveldb\helpers\memenv;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>