Update some basic text for 2026

[strugee]

Definitely more to do here, but this at least gets the dates for next year up.

[altsalt]

made a bunch of other small changes

question, I saw some logic in feed.xml that seems to be relevant to the year 2024-02-11, can we confirm?

here are the remaining changes I noticed we should do as available:

• update custom.url.prospectus
• change schedule.md redirect
• update the visible links.yml
• replace header.html get_involved with cfp then schedule link
  • maybe this can be accomplished via a conditional?
  • ea74536
  • 688de36
• update header.html sponsors link
• add 2026 fliers
• update 2026 maps