Task: replace all mentions of targets with artifacts in all messages to the user

[MVrachev]

What is the task about?

Replace all mentions of targets with artifacts in all messages to the user.
A good example of this inconsistency is when we add:

repository-service-tuf-worker/repository_service_tuf_worker/repository.py

Line 884 in 3130543

message="Target(s) Added",

or remove a targets:

repository-service-tuf-worker/repository_service_tuf_worker/repository.py

Line 960 in 3130543

message="Target(s) removed",

Find all places where we mention targets and ensure we don't mention that word to the user when we speak about target files.

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