Rails5: fix mysql milliseconds problem in project_spec

What does this MR do?

Rails 5 mysql milliseconds fix in ee/spec/models/project_spec.rb. Similar to gitlab-ce!20221 and gitlab-ce!20222.

Fixes specs:

rspec ./ee/spec/models/project_spec.rb:55
rspec ./ee/spec/models/project_spec.rb:70
rspec ./ee/spec/models/project_spec.rb:84
rspec ./ee/spec/models/project_spec.rb:379
rspec ./ee/spec/models/project_spec.rb:391

See https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/88585619

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?