Upgrade target frameworks

This commit updates the project to use the latest .NET framework version
and removes any previously unsupported framework versions. This ensures
compatibility with the most recent features, performance improvements,
and security updates.

Author: Gideon de Swardt

src/ExCSS.Tests/ExCSS.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net5.0</TargetFramework>
+    <TargetFramework>net7.0</TargetFramework>
     <AssemblyName>ExCSS.Tests</AssemblyName>
     <PackageId>ExCSS.Tests</PackageId>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>

src/ExCSS/ExCSS.csproj

@@ -2,7 +2,7 @@
 
 	<PropertyGroup>
 		<LangVersion>9.0</LangVersion>
-		<TargetFrameworks>net5.0;netstandard2.0;netcoreapp3.1;net452</TargetFrameworks>
+		<TargetFrameworks>net7.0;net6.0;netstandard2.1;netstandard2.0;netcoreapp3.1;net452</TargetFrameworks>
 		<AssemblyName>ExCSS</AssemblyName>
 		<PackageId>ExCSS</PackageId>
 		<Title>ExCSS .NET Stylesheet Parser</Title>