Remove unnecessary .NET SDK versions from PR workflow

[Copilot]

Description

The PR workflow was installing .NET SDK versions 5.0.x, 6.0.x, and 7.0.x which are not required. The project targets net462, net472, net48, net481, netstandard2.0, netstandard2.1, net8.0, and net9.0 - all of which can be built using only .NET 8.0.x and 9.0.x SDKs.

Type of change

• Refactor

How Has This Been Tested?

• Built the main library project targeting all frameworks (net462, net472, net48, net481, netstandard2.0, netstandard2.1, net8.0, net9.0) successfully with only .NET 8/9 SDKs installed

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional context

Reduces CI build time and resource usage by eliminating three unnecessary SDK installations.

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