Remove netstandard2.0 target framework from core packages

[rajkumar-rangaraj]

Removes netstandard2.0 from the following packages:

• Microsoft.ApplicationInsights
• Microsoft.ApplicationInsights.AspNetCore
• Microsoft.ApplicationInsights.WorkerService

These packages now target net8.0, net9.0, net10.0 (and net462 for the core SDK).

[Copilot]

Pull request overview

This PR removes the netstandard2.0 target framework from three Application Insights packages: Microsoft.ApplicationInsights, Microsoft.ApplicationInsights.AspNetCore, and Microsoft.ApplicationInsights.WorkerService. This simplifies the build targets and indicates these packages will no longer support .NET Standard 2.0, requiring users to target .NET 8.0 or higher (or .NET Framework 4.6.2 for the base package).

Changes:

• Removed netstandard2.0 from target frameworks in three project files
• Cleaned up conditional compilation directives and build configurations that referenced netstandard2.0
• Updated CHANGELOG.md to document the removal

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
NETCORE/src/Microsoft.ApplicationInsights.WorkerService/Microsoft.ApplicationInsights.WorkerService.csproj	Removed netstandard2.0 from TargetFrameworks, now targets only net10.0, net9.0, and net8.0
NETCORE/src/Microsoft.ApplicationInsights.AspNetCore/Microsoft.ApplicationInsights.AspNetCore.csproj	Removed netstandard2.0 from TargetFrameworks and removed the conditional TargetFrameworks for non-Windows OS, also removed the Release configuration PropertyGroup specific to netstandard2.0
BASE/src/Microsoft.ApplicationInsights/Microsoft.ApplicationInsights.csproj	Removed netstandard2.0 from both Windows and non-Windows TargetFrameworks, removed commented-out PackageReference, and updated ItemGroup condition to exclude only net8.0/9.0/10.0
CHANGELOG.md	Added entry documenting the removal of netstandard2.0 target framework from the three packages

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[Saibamen]

Why it is better to target direct versions instead of netstandard?

[rajkumar-rangaraj]

Why it is better to target direct versions instead of netstandard?

please review - https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-0#net-standard-problems

That being said, given the customer request and the fact that many netstandard apps have referenced it, we will include netstandard 2.0 in the next release in a couple of weeks.