Subscription Block: migration degrades performance

[obenland]

Working on Automattic/wp-calypso#40658 I noticed that the subscription block gets updated every time the page template selector gets loaded. It turns out the block gets run through the migration process every time it's loaded, regardless of whether the markup is current or not.

It looks like input and output are the same though, leading to an unnecessary performance degradation where the selector can't finish loading until every subscription block has been updated.

Steps to reproduce the issue

1. Go to https://wordpress.com/block-editor/page and select your test site.
2. Open your console in developer toolbar and find block update messages from subscription block

Screenshots

[apeatling]

@obenland This also seems to be true of core blocks from what I'm seeing in the console?