Update app.element.io configuration to migrate 10% of existing users to the new rust crypto stack

[BillCarsonFr]

Fixes #27002

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Update app.element.io configuration to migrate 10% of existing users to the new rust crypto stack (#27032). Fixes Element R: Automatically Migrate 10% of existing users on app.element.io and Element Desktop stable (and 100% on Element Desktop nightly) to rust sdk #27002.

[BillCarsonFr]

Merge blocked by #26967

[fnwbr]

You might also wanna be aware of #27165 before rolling this out.