Updates on Self-Serve Upgrade

@begriffs,

We made some significant changes and improvements on self-serve upgrade features in Citus Cloud based on the feedback we gathered from our customers. At the moment, documentation about the self-serve upgrade is outdated and needs to be updated.

Here are key highlights;
- You can now **Schedule** an upgrade for a future time. Target time must be at least 24 hour ahead (to allow creating standby servers and replaying WAL)
- If there are more than 24 hour to upgrade time, you can **Reschedule** or **Cancel** upgrade operation.
- If your standby servers are ready before your target upgrade time (which is most of the time anyway), an **Upgrade Now** buttons appears which lets you upgrade your formation without waiting to upgrade time.