Use new ButtonLink prop name

Author: hypha

src/content/pages/sponsor.mdx

@@ -21,7 +21,8 @@ Join us from July 14th to 20th in the **[Prague Congress Centre (PCC)](https://w
 #### Ready to Become a EuroPython 2025 Sponsor?
 <div class="text-center mt-4">
 
-<ButtonLink href="undefined">Sign Up Now!</ButtonLink>
+<ButtonLink url="undefined">Sign Up Now!</ButtonLink>
+
 </div>
 
 
@@ -31,7 +32,7 @@ EuroPython 2024 was a hybrid conference, hosted both in Prague and online,
 bringing together the best of both worlds.
 
 <div class="text-center mt-4">
-<ButtonLink href="undefined">EuroPython 2024 Stats & Demographics (PDF)</ButtonLink>
+<ButtonLink url="undefined">EuroPython 2024 Stats & Demographics (PDF)</ButtonLink>
 </div>
 
 ---
@@ -41,8 +42,8 @@ At EuroPython 2025, we offer eight main packages to keep EuroPython
 accessible to most organisations keen to help the community.
 
 <div class="text-left [&>*]:mr-8 [&>*]:mb-2 mb-12">
-<ButtonLink href="undefined">Summary of Sponsor Packages & Options (PDF)</ButtonLink>
-<ButtonLink href="undefined">Sign up now!</ButtonLink>
+<ButtonLink url="undefined">Summary of Sponsor Packages & Options (PDF)</ButtonLink>
+<ButtonLink url="undefined">Sign up now!</ButtonLink>
 </div>
 
 
@@ -62,8 +63,8 @@ add-ons as optional extras (undefined packages and onwards). Below are some exam
 - And more!
 
 <div class="text-left space-x-8">
-<ButtonLink href="undefined">Full list of Optional Extras (PDF)</ButtonLink>
-<ButtonLink href="undefined">Sign up now!</ButtonLink>
+<ButtonLink url="undefined">Full list of Optional Extras (PDF)</ButtonLink>
+<ButtonLink url="undefined">Sign up now!</ButtonLink>
 </div>
 
 Please note: each add-on has limited slots, and some are exclusive.