Resolve "Flaky test spec/models/release_highlight_spec.rb[1:1:1:1]"

What does this MR do?

Fix flaky tests related to memoization in ReleaseHighlights.file_paths.

ReleaseHighlights.file_paths memoizes the value in @file_paths class instance variable. When this class method is called across multiple tests, the values persist across tests.

When a test job runs spec/requests/whats_new_controller_spec.rb followed by spec/models/release_highlight_spec.rb, the test in the first file calls and memoizes @file_paths. When the latter tests from release_highlight_spec.rb calls .file_paths again, it uses the memoized values.

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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 #321526 (closed)