Upgrade to .NET 4.8.1 for native ARM64 support

It's been 2 years since release so plenty of time to upgrade.

DesktopClock.Tests/DesktopClock.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net48</TargetFramework>
+    <TargetFramework>net481</TargetFramework>
     <LangVersion>latest</LangVersion>
   </PropertyGroup>
 

DesktopClock/DesktopClock.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFramework>net48</TargetFramework>
+    <TargetFramework>net481</TargetFramework>
     <UseWPF>true</UseWPF>
     <LangVersion>latest</LangVersion>
     <ApplicationIcon>DesktopClock.ico</ApplicationIcon>