Update postbuild testing to .NET 8 (#1716)

I really need a more unified way of doing this....

Author: borrrden

src/Couchbase.Lite.Tests.NetCore/Couchbase.Lite.Tests.NetCore.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net462;net6.0</TargetFrameworks>
+    <TargetFrameworks>net462;net8.0</TargetFrameworks>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
     <LangVersion>latest</LangVersion>
   </PropertyGroup>