[`firestartr-bootstrap`] Improve JSON Schema for field `webhookUrl`

[juanjosevazquezgil]

Motivation

The field webhookUrl needs to be a full URL path (i.e., https://webhook.url.com). We have a JSON Schema created already, so we would need to update it to check the field's value is compilant with what the GitHub API wants

Acceptance criteria

• The JSON Schema now properly checks the webhookUrl field