Improve Spec Performance for Feature Flag Services

What does this MR do?

Refactor feature flag service specs to use let_it_be.

Contributes to #251358 (closed) and https://gitlab.com/gitlab-org/plan/-/issues/145.

Running FPROF=1 rspec spec/services/feature_flags/*_spec.rb gave the following improvements:

• Total amount of factories created went down from 218 to 66 🚀
• Total events went down from 8620 to 2361
• Queries saved: 6259

Before

Total time: 00:10.803 of 00:20.230 (53.4%)
Total events: 8620

Top 5 slowest suites (by time):

FeatureFlags::UpdateService (./spec/services/feature_flags/update_service_spec.rb:5) – 00:05.227 (4284 / 22) of 00:09.721 (53.77%)
FeatureFlags::CreateService (./spec/services/feature_flags/create_service_spec.rb:5) – 00:01.620 (1284 / 7) of 00:03.015 (53.74%)
FeatureFlags::EnableService (./spec/services/feature_flags/enable_service_spec.rb:5) – 00:01.469 (1219 / 9) of 00:02.711 (54.2%)
FeatureFlags::DestroyService (./spec/services/feature_flags/destroy_service_spec.rb:5) – 00:01.456 (1133 / 6) of 00:02.621 (55.56%)
FeatureFlags::DisableService (./spec/services/feature_flags/disable_service_spec.rb:5) – 00:01.029 (700 / 5) of 00:02.160 (47.67%)



Finished in 27.54 seconds (files took 2.45 seconds to load)
49 examples, 0 failures

Randomized with seed 15477

[TEST PROF INFO] Factories usage

 Total: 218
 Total top-level: 169
 Total time: 12.3449s
 Total uniq factories: 6

   total   top-level     total time      time per call      top-level time               name

      72          72        2.6775s            0.0372s             2.6775s               user
      49          49        8.2914s            0.1692s             8.2914s            project
      49           0        2.0746s            0.0423s             0.0000s          namespace
      36          36        1.3040s            0.0362s             1.3040s operations_feature_flag
       7           7        0.0405s            0.0058s             0.0405s operations_feature_flag_scope
       5           5        0.0316s            0.0063s             0.0316s            license

After

Total time: 00:02.932 of 00:05.905 (49.66%)
Total events: 2361

Top 5 slowest suites (by time):

FeatureFlags::UpdateService (./spec/services/feature_flags/update_service_spec.rb:5) – 00:01.541 (992 / 22) of 00:02.997 (51.43%)
FeatureFlags::EnableService (./spec/services/feature_flags/enable_service_spec.rb:5) – 00:00.431 (428 / 9) of 00:00.844 (51.02%)
FeatureFlags::DestroyService (./spec/services/feature_flags/destroy_service_spec.rb:5) – 00:00.382 (340 / 6) of 00:00.682 (56.08%)
FeatureFlags::CreateService (./spec/services/feature_flags/create_service_spec.rb:5) – 00:00.309 (337 / 7) of 00:00.835 (37.1%)
FeatureFlags::DisableService (./spec/services/feature_flags/disable_service_spec.rb:5) – 00:00.266 (264 / 5) of 00:00.544 (48.99%)



Finished in 13.5 seconds (files took 2.66 seconds to load)
49 examples, 0 failures

Randomized with seed 25398

[TEST PROF INFO] Factories usage

 Total: 66
 Total top-level: 61
 Total time: 3.0531s
 Total uniq factories: 6

   total   top-level     total time      time per call      top-level time               name

      36          36        1.3469s            0.0374s             1.3469s operations_feature_flag
       8           8        0.3164s            0.0396s             0.3164s               user
       7           7        0.0383s            0.0055s             0.0383s operations_feature_flag_scope
       5           5        0.0831s            0.0166s             0.0831s            license
       5           5        1.2683s            0.2537s             1.2683s            project
       5           0        0.2805s            0.0561s             0.0000s          namespace

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