Fix flakey spec for applicable to branch
What does this MR do and why?
Fixes flakey test for applicable_to_branch tests, where modified_from_project_rule is set up on update, so instead remove the update in the spec, and put those attributes in the create method instead.
Related to: #394838 (closed)
To test:
bundle exec rspec ee/spec/models/merge_request_spec.rb:210:104 --seed 60655 --format documentation
Edited by Marc Shaw