feat(artifact): add support for never keyword in expire_in

What does this MR do?

This MR adds the possibility to use a never keyword as artefact expire_in value for a gitlab-ci.yml configuration to keep artefacts forever. It has the same effect as pressing the Keep Forever button on the pipeline view. E.g:

pages:
  script:
    - echo '...'
  artifacts:
    expire_in: never
    paths:
      - public

While on gitlab.com, the default expiry-time is set to nil, it is by default 30 days for self hosted instances. A default expiry-time is a really nice feature, but it makes it impossible to keep an artefact forever without using the Keep Forever button on the UI or using bogus expire_in values like 9999 years. This makes the use of GitLab as release artefact store impractical.

Closes #22761 (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

backend: Since the never keyword shall only be valid during the validation and artefact creation/upload, I've added it only to those specific function blocks. This keeps the changes quite small, but it might not be the most ideal solution.

• 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