feature: Migrate RestSharp to target .NET Framework 4.8

[devin-ai-integration]

Summary

Retargets the RestSharp core library and test projects from .NET 3.5/4.0 to .NET Framework 4.8 and removes the legacy RestSharp.Net2 project, which existed only to backport features (LINQ + System.Xml.Linq) that ship in 4.8. Verified with mono xbuild (all projects build) and the xUnit console runner: 83/83 unit tests pass.

Project retargeting

• RestSharp.csproj: v3.5 → v4.8; removed <TargetFrameworkProfile>Client</TargetFrameworkProfile> (no Client Profile in 4.x), removed the <RequiredTargetFramework>3.5</RequiredTargetFramework> hints on System.Core / System.ServiceModel.Web / System.Xml.Linq, dropped the 3.5-SP1 BootstrapperPackage entries, and removed the duplicate Microsoft.CSharp.targets import ($(MSBuildBinPath) one; kept $(MSBuildToolsPath)). DefineConstants left as TRACE;DEBUG;FRAMEWORK.
• RestSharp.Tests.csproj / RestSharp.IntegrationTests.csproj: v4.0 → v4.8.

Dependency upgrades

• Newtonsoft.Json 4.x → 13.0.3 (lib\net45), reference + packages.config + nuspec dependency range [13.0.3,).
• xUnit 1.9.x → 2.4.2. References switched to xunit.core / xunit.assert / xunit.abstractions (+ xunit.execution.desktop, required so the v2 console runner can discover/run the desktop test assembly). Test sources needed no code changes (only Assert.*/[Fact] are used; no PropertyData/DoesNotThrow/xunit.extensions).

Removed RestSharp.Net2 project

• Deleted RestSharp.Net2/ (LinqBridge + 28-file System.Xml.Linq reimplementation) and its packages.config; removed the project from both RestSharp.sln and the renamed RestSharp.Net2.sln → RestSharp.Net48.sln; dropped its entry from packages/repositories.config.

Conditional compilation cleanup

The NET_2_0/Net2 symbols are now never defined, so the dead branches were removed (kept FRAMEWORK blocks, which still distinguish Silverlight/WP/Mono targets):

-#if NET_2_0
-[assembly: AssemblyCompany(".../RestSharp-.NET-2.0-Fork")]
-#else
 [assembly: AssemblyCompany("restsharp.org")]
 [assembly: AssemblyCopyright("Copyright © RestSharp Project 2009-2011")]
-#endif

-#if Net2
-using RestSharp.Contrib;
-#endif

(Only these two files in this fork actually referenced NET_2_0/Net2; the broader file list in the request reflects upstream RestSharp, not this fork.)

Notes / out of scope

• The other platform projects (Silverlight, WindowsPhone, Mono*) and RestSharp.Mono.sln were left untouched — they are separate platform targets, not part of the 4.8 retarget.
• Integration tests rely on live/local HTTP servers and were not run as part of the unit-test gate.
• The committed packages/ folder (the repo vendors its NuGet packages) was updated: added the Newtonsoft 13.0.3 + xUnit 2.4.2 package set and removed the now-unused 4.x/1.x packages.

Test plan

xbuild /p:Configuration=Release RestSharp/RestSharp.csproj
xbuild /p:Configuration=Release RestSharp.Tests/RestSharp.Tests.csproj
xbuild /p:Configuration=Release RestSharp.IntegrationTests/RestSharp.IntegrationTests.csproj
mono packages/xunit.runner.console.2.4.2/tools/net452/xunit.console.exe \
  RestSharp.Tests/bin/Release/RestSharp.Tests.dll -parallel none
# => Total: 83, Errors: 0, Failed: 0, Skipped: 0

Link to Devin session: https://app.devin.ai/sessions/29818d0169444e82a3eca9a1f16d0ae6
Requested by: @travismyers-png

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