.Net: Cleanup of SKEXP0020 flag, removing MemoryStore samples and stop publishing DuckDB and Kusto connectors.

[westey-m]

Motivation and Context

• We are suppressing the SKEXP0020 experimental flag everywhere, but it is mostly not being used anymore, so removing unnecessary suppressions.
• Also removing / replacing any MemoryStore samples, to stop advertising them, since it will be obsoleted in future.
• Stopping publishing the DuckDB and Kusto connectors, since we won't be building VectorStore implementations for them.

#10918
#8540

Description

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 😄