Expose degradation_threshold from CI config

What does this MR do?

Adds support for CI YAML variable named DEGRADATION_THRESHOLD. This will be used under browser performance testing like:

stages:
  - performance

include:
  template: Verify/Browser-Performance.gitlab-ci.yml

performance:
  variables:
    URL: https://example.com
    DEGRADATION_THRESHOLD: 5

When this variable is set, the performance metric won't report any degradation unless it exceeds the set amount of threshold.

Solves ##27599 (closed)

Screenshots

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

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