.Net: Fix weaviate collection name checking

[stratosblue]

Motivation and Context

According to the weaviate document, the collection name allows the use of underscores

• collections: The collection name validation regex is /^[A-Z][_0-9A-Za-z]*$/.

Description

Allow _ in weaviate collection name.

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 😄