Fixed ":" char appearing which is not allowed in Windows paths

[JohnConnor123]

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ref: #182

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[sahel-sh]

Please change it within get_output_filename functions in all the rerankers in case they are/will be called from other places.
If you define a helper function (get_formatted_datetime or something similar) in the root RankLLM class, then all the get_output_filename implementations can call that.

Please also do a git grep to find datetime to see if we have this problem anywhere else, since ":" in the name comes from the datetime.isoformat that we append to the output files.