feat: make DNN configurable (clean up charm config)

[patriciareinoso]

Description

The AMF charm sets the supportDnnList value in the configuration file. This value is only used by the AMF if the UE's subscription information received from the UDM does not contain the default DNN (this is not the case in Charmed Aether SD-Core).

We are allowing the user to set custom DNN values using the UI. Keeping this field does not align with the dynamic nature of the configuration and could lead to confusion

See omec-project/amf#482

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