Change to_s(:medium) to to_fs(:medium)

What does this MR do and why?

Passing a format to to_s has been deprecated and replaced with to_fs in Rails 7.

This MR removes these deprecation warnings like this one DEPRECATION WARNING: TimeWithZone#to_s(:medium) is deprecated. Please use TimeWithZone#to_fs(:medium) instead..

https://github.com/rails/rails/blob/de7c495209811f8f918fa9f915e64df61a6080c1/guides/source/7_0_release_notes.md#deprecations-8

I think there are other formats in use, but I'm just changing the :medium format as that's the one I noticed while working on other issues and to keep this MR smaller.

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