userAgentPrefix doesn't use default policy

[YoDaMa]

• Package Name: @azure/iot-modelsrepository
• Package Version: 1.0.0-beta.1

Describe the bug
A clear and concise description of what the bug is.

From merge comment: #14863 (comment)

UserAgentPrefix currently uses custom defined default UserAgent prefix if customer does not provide one. Per @xirzec, "but it'd be good to use the same policy in case we change the format for clients later"

Creating this issue to track this change to be implemented for next beta release.

[github-actions]

Hi @YoDaMa, we deeply appreciate your input into this project. Regrettably, this issue has remained inactive for over 2 years, leading us to the decision to close it. We've implemented this policy to maintain the relevance of our issue queue and facilitate easier navigation for new contributors. If you still believe this topic requires attention, please feel free to create a new issue, referencing this one. Thank you for your understanding and ongoing support.