Use let_it_be in projects/issues_controller_spec

What does this MR do?

It replaces let with let_it_be in spec/controllers/projects/issues_controller_spec.rb

• Related issue: https://gitlab.com/gitlab-org/plan/-/issues/145
• Queries saved: 19510
• Saved time: 23s (50%)

Before

EventProf results for sql.active_record

Total time: 00:45.348 of 02:36.236 (29.03%)
Total events: 41490

After

EventProf results for sql.active_record

Total time: 00:22.553 of 01:09.973 (32.23%)
Total events: 21980

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