Rename Uptime Alerts back to Uptime Monitors

### Problem Statement

Uptime product team recently ranamed "Uptime Monitors" to "Uptime Alerts" in the product and in the documentation.

"Uptime Alerts" do not trigger any alerts nor send any notifications to users. To trigger an alert users need to create an actual "Issue Alert".

### Solution Brainstorm

Rename it back to Uptime Monitors, because this is what they do: monitor the uptime of an endpoint.

Update the documentation to clearly state that a user needs to create an issue alert after creating an uptime monitor to receive notifications on downtimes.

### Product Area

Alerts

Start in the Alerts product area by locating the product and documentation occurrences of “Uptime Alerts.” Rename the relevant references to “Uptime Monitors” and update the documentation to explain that users must create an Issue Alert to receive downtime notifications; done means both the product wording and documentation are consistent.

Written by the indexing model from the issue text.