doc: Update GSoC reference in README

Just so there's no confusion about whether this is true again
this year.

(The substance of the project description lives at the linked page,
in the zulip/zulip repo, and already got updated last week.)

Author: gnprice

README.md

@@ -27,10 +27,10 @@ You can also [try out this beta app][beta].
 Contributions to this app are welcome.
 
 If you're looking to participate in Google Summer of Code with Zulip,
-this is one of the projects we're [accepting GSoC 2024 applications][]
+this is one of the projects we intend to accept [GSoC 2025 applications][gsoc]
 for.
 
-[accepting GSoC 2024 applications]: https://zulip.readthedocs.io/en/latest/outreach/gsoc.html#mobile-app
+[gsoc]: https://zulip.readthedocs.io/en/latest/outreach/gsoc.html#mobile-app
 
 
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