Update Miniscript.csproj

Let's try targeting ONLY .net 6.0

Author: JoeStrout

MiniScript-cs/Miniscript.csproj

@@ -4,7 +4,7 @@
     <OutputType>Exe</OutputType>
     <RootNamespace>Miniscript</RootNamespace>
     <AssemblyName>Miniscript</AssemblyName>
-    <TargetFrameworks>net45;net5.0;net6.0;net7.0</TargetFrameworks>
+    <TargetFrameworks>net6.0</TargetFrameworks>
     <PackageId>org.miniscript.MiniScript</PackageId>
     <VersionPrefix>1.6.2</VersionPrefix>
     <Authors>Joe Strout</Authors>