fix: Use the right MTU for the common use case by default

[ghislainbourgeois]

Description

The previous MTU of 1460 was just a bit too big for the most common setup of running over Ethernet with a standard MTU of 1500. Changing the default to 1456 prevents the UE from sending packets that would need fragmentation.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.