Speed up banzai milestone reference filter specs

What does this MR do?

This MR improves the performance of bazai milestone reference filter specs by using let_it_be.

Related to &3752.

Contributes to https://gitlab.com/gitlab-org/plan/-/issues/145.

Here's the output from test-prof when run with FPROF=1. Note:

• Total amount of factories created went down from 771 to 40 🚀
• Total events went down from 14171 to 2259
• Queries saved: 11912

Before

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:25.496 of 00:52.190 (48.85%)
Total events: 14171

Top 5 slowest suites (by time):

Banzai::Filte...eferenceFilter (./spec/lib/banzai/filter/milestone_reference_filter_spec.rb:5) – 00:25.496 (14171 / 131) of 00:52.190 (48.85%)



Finished in 57.63 seconds (files took 3.86 seconds to load)
131 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 771
 Total top-level: 539
 Total time: 47.6946s
 Total uniq factories: 6

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

     227         227        8.6932s            0.0383s             8.6932s              group
     156         156       32.0381s            0.2054s            32.0381s            project
     140          24        8.4696s            0.0605s             1.8312s          namespace
     131         131        5.1158s            0.0391s             5.1158s          milestone
     116           0        5.6453s            0.0487s             0.0000s               user
       1           1        0.0164s            0.0164s             0.0164s            license

After

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:03.619 of 00:08.929 (40.53%)
Total events: 2259

Top 5 slowest suites (by time):

Banzai::Filte...eferenceFilter (./spec/lib/banzai/filter/milestone_reference_filter_spec.rb:5) – 00:03.619 (2259 / 131) of 00:08.929 (40.53%)



Finished in 14.33 seconds (files took 3.9 seconds to load)
131 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 40
 Total top-level: 34
 Total time: 3.2971s
 Total uniq factories: 6

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

      15          15        0.6101s            0.0407s             0.6101s          milestone
       9           9        0.8315s            0.0924s             0.8315s              group
       8           8        1.8061s            0.2258s             1.8061s            project
       4           1        0.4131s            0.1033s             0.0365s          namespace
       3           0        0.1152s            0.0384s             0.0000s               user
       1           1        0.0131s            0.0131s             0.0131s            license

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