Add `user_preferences` table into the list of `HighTrafficTables`

What does this MR do?

Add user_preferences table into the list of `HighTrafficTables

user_preferences table has the same size and closer access patterns to the users table. I think it could be considered high-traffic in gitlab.com scale.

gitlabhq_production=> select count(*) from users;
  count  
---------
 7400979

gitlabhq_production=> select count(*) from user_preferences;
  count  
---------
 5052939

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