Update Avalonia.ThemeManager.UnitTests.csproj

tests/Avalonia.ThemeManager.UnitTests/Avalonia.ThemeManager.UnitTests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net7.0</TargetFramework>
     <OutputType>Library</OutputType>
     <GeneratePackageOnBuild>False</GeneratePackageOnBuild>
     <Nullable>enable</Nullable>