fix: workaround for edit NS bug

[patriciareinoso]

Description

Modifying the MNC or the MCC fields of a Network Slice creates a duplicate subscriber (unexpected behavior). omec-project/webconsole#200

This workaround disables the possibility of editing the MNC and MCC fields of a network slice.

Editing the gNB or the UPF works fine.

Edit Network Slice Modal:

Create Network Slice Modal: (no changes here)

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.