Upgrade target frameworks to net10.0 and latest C#

[kblok]

Description

Update all .csproj files to target net10.0, upgrade NuGet packages (xunit 2.9+, Microsoft.NET.Test.Sdk 17.x), remove net48/netcoreapp3.1 targets, and update LangVersion to latest.

What to Implement

• Update TargetFramework in all .csproj files from netcoreapp3.1/net48/netstandard2.0 to net10.0 (or netstandard2.1 for Abstractions)
• Update LangVersion to latest or 13 across all projects
• Upgrade NuGet packages:
  • xunit to 2.9+
  • Microsoft.NET.Test.Sdk to 17.x
  • Other dependencies to latest compatible versions
• Fix any breaking changes from the framework upgrade
• Ensure the solution builds cleanly

Files to Modify

• src/PlaywrightSharp.Abstractions/PlaywrightSharp.Abstractions.csproj
• src/PlaywrightSharp.Chromium/PlaywrightSharp.Chromium.csproj
• src/PlaywrightSharp.Firefox/PlaywrightSharp.Firefox.csproj
• src/PlaywrightSharp.Webkit/PlaywrightSharp.Webkit.csproj
• src/PlaywrightSharp.Tests/PlaywrightSharp.Tests.csproj
• src/PlaywrightSharp.TestServer/PlaywrightSharp.TestServer.csproj
• Any other .csproj files in the solution

Verification

dotnet build ./src/PlaywrightSharp.sln
dotnet test ./src/PlaywrightSharp.Tests/PlaywrightSharp.Tests.csproj --filter Category=chromium -c Debug -f net10.0

Dependencies

None — this is the foundational issue.