Rename Talk `published_at` to `date` or similar

[marcoroth]

I understood published_at as "the date the video was published on", but something like date would probably be more accurate.

Originally posted by @marcoroth in https://github.com/adrienpoly/rubyvideo/issues/113#issuecomment-2293704029

[adrienpoly]

I agree we should use date so that it is less confusing

in #118 I started to look first for date and then as a fallback published_at