Use `let_it_be` for Static Site Editor test suite

What does this MR do?

It updates Static Site Editor tests to use let_it_be (according to best practices)

It provides a small performance improvement too

bundle exec rspec spec/controllers/projects/static_site_editor_controller_spec.rb spec/features/static_site_editor_spec.rb spec/requests/api/graphql/mutations/branches/create_spec.rb

• Before: Finished in 41.74 seconds (files took 38.35 seconds to load)
• After: Finished in 37.67 seconds (files took 37.72 seconds to load)

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