deprecate rds_prefix option in process_spectra() because of {targets}

[cpauvert]

• does not necessary make sense if the workflow in handled by {targets}
• could be better file format alternative (e.g., qs)
• Look at: https://lifecycle.r-lib.org/articles/communicate.html to do it properly, (EDIT: more specifically for arguments: https://lifecycle.r-lib.org/articles/communicate.html#deprecating-an-argument-providing-a-new-default)
• will get rid of untested sections of the function (https://app.codecov.io/github/ClavelLab/maldipickr/blob/main/R%2Fprocess_spectra.R) as the CRAN is not happy when writing to disk during tests.