Update default title of alerts to New: Alert

What does this MR do?

Related issue: #299046 (closed)

When an alert is sent to gitlab without a title, we should name the alert as 'New: Alert'; when an incident is created from an alert, the title should be changed from 'New: Alert' to 'New: Incident' instead, preserving the added 'iid' for the incident in the title.

Screenshots

Testing:

1. Create a new HTTP Integration under Settings > Operations > Alerts
2. Send a test alert with an empty payload
3. See the alert under Operations > Alert
4. Create an incident
5. See the incident title

New alert	New incident from alert
Manual issue creation:
Automatic issue creation:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team