🧹 [Maintenance]: Add `logger.Output` -> `logger.Stream` change to whats_new.md

[grivera64]

Maintenance Task Description

The change from logger.Output to logger.Stream is not documented in the whats_new.md page. We can add a brief segment that mentions this change under the logger section of docs.gofiber.io.

Impact on the Project

By documenting this change to the logger middleware, it makes it easier for users to see what they need to change to migrate to v3.

Additional Context (optional)

This change was made in #3333.

We can also make a separate issue to add a regex that updates this in the fiber cli migration tool

Checklist:

• I have confirmed that this maintenance task is currently not being addressed.
• I understand that this task will be evaluated by the maintainers and prioritized accordingly.
• I am available to provide further information if needed.

[ReneWerner87]

gofiber/cli#269 migratior