Port dotnet feature: .Net: Delete CreateCollection, rename CreateIfNotExists and make EnsureExists thread safe.

[github-actions]

Original issue

#11937

Description

Motivation and Context

#11829

Description

• DeleteCreateCollection
• Rename CreateCollectionIfNotExists to EnsureCollectionExists
• Make EnsureCollectionExists thread safe
• Update tests and samples.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

\n Relates to #11937