chore: Change the interface name from fiveg-nrf to fiveg_nrf

[gatici]

Description

NRF and AMF use different relation names for fiveg_nrf interface. All the other charms uses fiveg_nrf. We need to use the same naming convention in all charms. For this reason interface name is changed from fiveg-nrf -> fiveg_nrf within this PR.

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
• I have bumped the version of the library