fix: update REPL embed version to Svelte 5

[Pika-pikachu30]

Fixes #1968

The blog post example uses Svelte 5 syntax, but the embedded REPL was loading Svelte 3 (?version=3).

This updates the embed URL to use Svelte 5 (?version=5) so the REPL matches the example code.

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[manvankaliber]

As I just commented in the issue, shouldn't the code snippet be reverted back to Svelte 3/4 syntax instead?

Full comment here: #1968 (comment)

[Pika-pikachu30]

I'd say that reverting to Svelte 3/4 code wouldn't be the right move becuase the decision to use Svelte 5 syntax in this blog post was already made in PR #294, which migrated examples across the site and also that PR was merged by the maintainers. thats why i think we should change here also to Svelte 5.