[bisq-network/bisq] Clean up AccountAgeWitnessService (#3781)

[sqrrm]

### Description
There are inconsistencies in how the different time spans are used.

`SIGNER_AGE_DAYS` in `SignedWitnessService` is used to decide age of a signer but the enums in `AccountAgeWitnessService` are used for user presentation and they don't agree.

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