Remove "Congratulations to all Astropy contributors" after 2025-01-17

[pllim]

This is a follow-up of #621 where @taldcroft said the top "Congratulations to all Astropy contributors" can be removed after "2 or 3 months". It was merged on 2024-09-23.

Also remove the banner added in #636 (if it is merged). That is, empty the content but do not remove the file.

[eteq]

Now that I think about it, how about we leave this in until after the AAS talk? Since that's specifically what the prize is about. So it can go away 2025-1-17.