Change history retention window to instant restore

[bgrenon]

The settings page in the UI has changed to Instant Recovery Window.

Let's settle on instant restore as the name for this feature. TBD to use window in the settings, title or not.

Fix docs to use instant restore period/window instead of history retention period.

cc Dan when ready to make the change; will do a console PR at the same time.

[danieltprice]

Alex has opened this PR:
https://github.com/neondatabase/cloud/pull/26356