.Net: Replaced IMemoryStore with IVectorStore in examples

[dmytrostruk]

Motivation and Context

Replaced IMemoryStore usage with IVectorStore in Semantic Kernel examples.

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 😄

[RogerBarreto]

Suggest changing the example to check if the apikey is set in the secrets and use it instead. So this can work for customers and internal testing at the same time.

Similar on how I did it here:

semantic-kernel/dotnet/samples/Demos/AIModelRouter/Program.cs

Line 57 in 8dadef2

if (config["AzureOpenAI:ApiKey"] is not null)

Suggested change

	new AzureCliCredential());
	new AzureCliCredential());