Rename Tails Greeter to be more plain

During the expert review, “Tails Greeter” wasn’t an intuitive name. She would have preferred “Tails Welcome Screen” or something similar.

I understand that “Greeter” comes from the backend technology from GNOME.

We could check how other projects are calling such screens.

Parent Task: #14548

Original created by @sajolida on 15122 (Redmine)

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