chore: Updates Atlas Go SDK

[svc-apix-Bot]

Automatic update for MongoDB Atlas Go Client SDK

[augmentcode]

🤖 Augment PR Summary

Summary: This PR performs an automated bump of the MongoDB Atlas Go SDK used by the CloudFormation resources.

Changes:

• Updates the Atlas SDK module dependency from v20250312013 to v20250312020 in cfn-resources/go.mod
• Updates all Go imports that reference the Atlas SDK admin package to the new versioned module path
• Updates test imports accordingly, including the versioned mockadmin package used in unit tests
• Touches both resource handlers/mappings and associated unit/e2e tests to keep them consistent with the new SDK path

Technical Notes: The change is primarily a module path/version update; no functional logic changes are introduced beyond adopting the newer SDK package version.

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[augmentcode]

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