Subscriptions: post-subscribe redirection behavior no longer customizable

Prior to Jetpack 12.2, specifically #28170, when submitting the Subscription block, the submission went through `Jetpack_Subscriptions`'s `widget_submit` method:

https://github.com/Automattic/jetpack/blob/4fb690625a04c22b689a610a379081858e1780d9/projects/plugins/jetpack/modules/subscriptions.php#L102-L105

This process handles the submission and ends with a redirect that can be customized via a filter:

https://github.com/Automattic/jetpack/blob/4fb690625a04c22b689a610a379081858e1780d9/projects/plugins/jetpack/modules/subscriptions.php#L768-L779

In practice, that means you could do something like this to redirect folks to a "Thank you" page upon submission, for example:

```php
/**
* Redirect all successful subscription submissions to a 'thank-you' page
*/
function jeherve_custom_sub_redirect_page( $result ) {
if ( 'success' === $result ) {
$thanks_page = 'thank-you';
wp_safe_redirect( $thanks_page );
exit;
}
}
add_action( 'jetpack_subscriptions_form_submission', 'jeherve_custom_sub_redirect_page' );
```

This no longer works now, since the submission process reloads the same page instead:

https://github.com/Automattic/jetpack/blob/94ddae8284758db97529fad30752b9e11a556bb1/projects/plugins/jetpack/extensions/blocks/subscriptions/view.js#L52-L55

It would be nice if the existing filter could continue to work like before, taking over the submission process when it is set.

Compare the historical Jetpack_Subscriptions::widget_submit flow and its redirect filter in projects/plugins/jetpack/modules/subscriptions.php with the current submission handling in projects/plugins/jetpack/extensions/blocks/subscriptions/view.js. Start by tracing where the block reloads the page; done means the existing jetpack_subscriptions_form_submission action can again take over successful submissions and redirect to a custom destination.

Written by the indexing model from the issue text.