[BP-1232] Add test coverage for LdapConnectionPool excluded domains

[definitelynotagoblin]

Description

And correct use of dotnet preview feature use on LdapUtils

Motivation and Context

https://specterops.atlassian.net/browse/BP-1232

Adding test coverage for excluded domains, and correcting the use of a preview feature in LdapUtils that can cause build trouble.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Chore (a change that does not modify the application functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Documentation updates are needed, and have been made accordingly.
• I have added and/or updated tests to cover my changes.
• All new and existing tests passed.
• My changes include a database migration.