Use with let_it_be in features/boards_spec

What does this MR do?

Replace let with the helper let_it_be in spec/features/boards/boards_spec.rb to reduce the number of DB queries.

Queries saved: 14080

• Saved time: 16 seconds (44.44%)

Before

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:36.242 of 04:36.400 (13.11%)
Total events: 30008

After

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:19.762 of 03:44.260 (8.81%)
Total events: 15928

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