Resolve "Use "billing" feature category for Seat Link workers"

What does this MR do?

This MR updates the feature categories for the Seat Link workers (SyncSeatLinkWorker and SyncSeatLinkRequestWorker) to use the new billing category. This category was recently added to the handbook. This category replaces the existing :analysis category in these workers as it is associated with the Fulfillment team.

As part of this work, I ran scripts/update-feature-categories to update config/feature_categories.yml. More info about that process can be found here.

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

Closes #215199