Fix pagerduty_integration to include severity field

[joeldavidw]

Updates pagerduty integration to require the severity field

severity can have one of the following values:

• info
• warning
• error
• critical

Currently, when attempting to create a new PagerDuty integration, the process fails with the following error:

╷
│ Error: POST https://uptime.betterstack.com/api/v2/pager-duty-webhooks returned 422: {"errors":{"severity":["can't be blank"]}}
│ 
│   with betteruptime_pagerduty_integration.pagerduty_integration["XXXX"],
│   on pagerduty_integrations.tf line 12, in resource "betteruptime_pagerduty_integration" "pagerduty_integration":
│   12: resource "betteruptime_pagerduty_integration" "pagerduty_integration" {
│ 
╵