Halve complexity multiplier on BoardListIssues

What does this MR do?

At the moment we are porting over Issue Boards to use GraphQL &3900 (closed). Since Issue Boards are quite complex, the risk is that the frontend might bump into our query complexity limit, which is there to prevent malicious or overenthusiastic queries from taking up too many resources.

This MR halves the complexity multiplier for issues only being exposed via Issue Board Lists until we get a better handle on the query complexity required by the frontend and the pagination for issues #230403 (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
• 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

Related to #235107 (closed)