[Self-hosted] Cached notifications are retained after logging out of Jetpack

[twstokes]

Expected behavior

Locally cached notifications to be flushed when logging out of Jetpack

Actual behavior

Cached notifications aren't flushed and break the notifications view

Steps to reproduce the behavior

1. Connect the app to a self-hosted site with Jetpack installed and activated
2. Tap the Notifications tab and observe existing notifications
3. Log out of WordPress.com from the app
4. The notifications screen should now show a message "Looks like you have Jetpack set up on your site. Congrats!..." with a "Log in" button. There may be a large gap above it.
5. Scrolling down in the notifications screen shows notifications previously received when logged in, and some may be visually cut off

Large gap	Old notifications

Tested on iPhone 12, iOS 15.5, WPiOS 20.0