Drop support for obsolete runtimes

- remove .net 4.5.2 (end of life: January 2023)
- upgrade to .net 4.6.2 (end of life: January 2027)
- remove .net core 3.1 (end of life: December 2022)
- upgrade to .NET 6 (LTS, end of life: November 2024)

Author: Kryptos-FR

src/Markdig.Wpf.SampleApp/Markdig.Wpf.SampleApp.csproj

@@ -7,7 +7,7 @@
     <AssemblyTitle>Markdig.Wpf.SampleApp</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.5.0.1</VersionPrefix>
-    <TargetFrameworks>net452;netcoreapp3.1;net5.0-windows</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0-windows</TargetFrameworks>
     <ApplicationIcon />
     <OutputType>WinExe</OutputType>
     <StartupObject />

src/Markdig.Wpf.SampleAppCustomized/Markdig.Wpf.SampleAppCustomized.csproj

@@ -7,7 +7,7 @@
     <AssemblyTitle>Markdig.Wpf.SampleAppCustomized</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.5.0.1</VersionPrefix>
-    <TargetFrameworks>net452;netcoreapp3.1;net5.0-windows</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0-windows</TargetFrameworks>
     <ApplicationIcon />
     <OutputType>WinExe</OutputType>
     <StartupObject />

src/Markdig.Wpf/Markdig.Wpf.csproj

@@ -7,7 +7,7 @@
     <AssemblyTitle>Markdig.Wpf</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.5.0.1</VersionPrefix>
-    <TargetFrameworks>net452;netcoreapp3.1;net5.0-windows</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0-windows</TargetFrameworks>
     <SignAssembly Condition="'$(Configuration)' == 'Release_Signed'">true</SignAssembly>
     <PackageId>Markdig.Wpf</PackageId>
     <PackageId Condition="'$(SignAssembly)' == 'true'">Markdig.Wpf.Signed</PackageId>

src/Markdig.Xaml.ConsoleApp/Markdig.Xaml.ConsoleApp.csproj

@@ -7,7 +7,7 @@
     <AssemblyTitle>Markdig.Xaml.ConsoleApp</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.5.0.1</VersionPrefix>
-    <TargetFrameworks>net452;netcoreapp3.1;net5.0-windows</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0-windows</TargetFrameworks>
     <ApplicationIcon />
     <OutputType>Exe</OutputType>
     <StartupObject />

src/Markdig.Xaml.SampleApp/Markdig.Xaml.SampleApp.csproj

@@ -7,7 +7,7 @@
     <AssemblyTitle>Markdig.Xaml.SampleApp</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.5.0.1</VersionPrefix>
-    <TargetFrameworks>net452;netcoreapp3.1;net5.0-windows</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0-windows</TargetFrameworks>
     <ApplicationIcon />
     <OutputType>WinExe</OutputType>
     <StartupObject />