Update the buy tickets link

[jaipurandare]

Change to from 2024 to 2025

it should be : https://pretix.eu/bapya/pybay-2025/

we need to update the link to buy tickets since it points to the site for 2024, and the verbage around it. (specifically need to take out the language on "still shows a “Buy tickets now, speakers!” admonition that links to tickets for PyBay 2024")

The link should be: https://pretix.eu/bapya/pybay-2025/ ...which will pull up a standard error page until we allow ticket purchases to start -- but that's better than linking to last year's site.