Update target frameworks to netstandard2.0;netstandard2.1;net8.0;net10.0

[gavinbarron]

Summary

Update the target framework monikers (TFMs) for the SDK and test projects:

• Microsoft.Graph: netstandard2.0;netstandard2.1;net5.0 → netstandard2.0;netstandard2.1;net8.0;net10.0
• Microsoft.Graph.DotnetCore.Test: net8.0 → net8.0;net10.0

Motivation

• net5.0 is end-of-life and should be dropped
• net8.0 (LTS) and net10.0 (current) should be targeted

Breaking Change

Dropping net5.0 is a breaking change for consumers who still target that framework.