Handle severity for generic payloads more gracefully

What does this MR do?

This MRs handles severity values in generic alert payloads more gracefully.

Will close #267376 (closed) and #297501 (closed).

Note, this MR moves Prometheus-specific code (introduced in !50871 (merged)) to Payload base class.

Severity Value Mapping

Severity is matched case-insensitive so one can provide CRITICAL or iNfO.

Generic Payload severity	Mapped to Alert severity now	before
CRITICAL	critical	errored
something unmapped	nil (or critical implicitly¹)	errored

¹If a severity is missing or unknown we map to nil which defaults to critical (DEFAULT 0)

Screenshots (strongly suggested)

Alerts

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) - !54070 (merged)
• 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