Add net462 target to ServiceDiscovery

[twsouthwick]

An add on from #10470 that added support for netstandard2.0, this adds
an explicit net462 target which is part of the recommendation for
multi-targetd libraries.

[Copilot]

Pull Request Overview

This PR adds explicit .NET Framework 4.6.2 (net462) targeting to the ServiceDiscovery libraries, complementing the existing netstandard2.0 and modern .NET targets. This follows Microsoft's recommendations for multi-targeted libraries to provide better compatibility with .NET Framework applications.

• Adds net462 to the TargetFrameworks property in both projects
• Updates AOT compatibility conditions to exclude net462 instead of just netstandard2.0
• Extends conditional package references and imports to include net462 alongside netstandard2.0

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Microsoft.Extensions.ServiceDiscovery.csproj	Adds net462 target and updates conditions for AOT compatibility, package references, and polyfill imports
Microsoft.Extensions.ServiceDiscovery.Abstractions.csproj	Adds net462 target and updates conditions for AOT compatibility, package references, and polyfill imports

[joperezr]

Looks good other than reversing the condition

[JamesNK]

Reason why: https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting

Using .NET Standard 2.0 from .NET Framework has some issues that were addressed in .NET Framework 4.7.2. You can improve the experience for developers that are still on .NET Framework 4.6.2 - 4.7.1 by offering them a binary that's built for .NET Framework 4.6.2.

4.6.2 is supported until 2027.