FX.Tests: move from net7.0 to net8.0

To fix this error when trying to run the tests:
```
Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
Testhost process for source(s) '/__w/FX/FX/src/FX.Tests/bin/Debug/net7.0/FX.Tests.dll' exited with error: You must install or update .NET to run this application.
App: /__w/FX/FX/src/FX.Tests/bin/Debug/net7.0/testhost.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '7.0.0' (x64)
.NET location: /usr/lib/dotnet/
The following frameworks were found:
  8.0.1 at [/usr/lib/dotnet/shared/Microsoft.NETCore.App]
Learn more:
https://aka.ms/dotnet/app-launch-failed
To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=7.0.0&arch=x64&rid=ubuntu.23.10-x64&os=ubuntu.23.10
. Please check the diagnostic logs for more information.

Test Run Aborted.
```

src/FX.Tests/FX.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net7.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
   </PropertyGroup>
 
   <ItemGroup>