Fix test target

Author: hadashiA

tests/Utf8StringInterpolation.Tests/Utf8StringInterpolation.Tests.csproj

@@ -1,10 +1,11 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>net48;net5.0;net6.0;net8.0</TargetFrameworks>
+		<TargetFrameworks Condition="'$(OS)' == 'Windows_NT'">net48</TargetFrameworks>
+		<TargetFrameworks>$(TargetFrameworks);net5.0;net6.0;net8.0</TargetFrameworks>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>
-		<LangVersion>11.0</LangVersion>
+		<LangVersion>12.0</LangVersion>
 		<IsPackable>false</IsPackable>
 		<IsTestProject>true</IsTestProject>
 	</PropertyGroup>