[ENH] ADNI-to-BIDS : `Visit` handling should be updated

[Adam-Ismaili-92]

Issues description
With the recent update of the ADNI-to-BIDS converter to support the new clinical data available on the IDA LONI platform, the logic used to handle visits is no longer up-to-date. In particular, the visits_to_timepoints function in the _visits_utils.py file relies on a column named Visit, which explicitly describes sessions/visits. This column is no longer present in the new clinical datasets, which instead rely solely on viscodes.

Desired solution
The visit-handling logic should be updated to consistently use viscodes instead of the deprecated Visit column. This would ensure compatibility with the new clinical data and provide a more robust and future-proof approach to visit and session management.

Additional context
The Visit column and the visits_to_timepoints function are also involved in the scan selection logic of the converter. As a result, any modification must be carefully reviewed and tested to avoid unintended impacts on scan selection and downstream processing.