CG-14164 Archive the .NET client SDK repository

[OptiKPS]

As a Graph engineer I want to archive the .NET SDK repository so that all viewers are aware of its deprecated status.

Acceptance Criteria

• A large note has been added to the top of the Read Me file indicating the repository is deprecated and no longer supported.
• The repository has been placed in an archive state that allows read-only access. The repository must not be deleted.

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[Grimmscorpp]

Looks good.