Refactor auto merge available_for methods

app/models/merge_request.rb

app/services/auto_merge/base_service.rb

...rvices/auto_merge/merge_when_pipeline_succeeds_service.rb

app/services/auto_merge_service.rb

...g/feature_flags/gitlab_com_derisk/refactor_auto_merge.yml

ee/app/models/ee/merge_request.rb

ee/app/services/auto_merge/merge_when_checks_pass_service.rb

ee/app/services/ee/auto_merge_service.rb

...ec/controllers/projects/merge_requests_controller_spec.rb

...er_approvals_reset_after_merging_to_source_branch_spec.rb

...eatures/merge_request/user_merges_with_push_rules_spec.rb

...features/merge_trains/two_merge_requests_on_train_spec.rb

ee/spec/requests/api/merge_trains_spec.rb

...ervices/auto_merge/merge_when_checks_pass_service_spec.rb

...c/services/merge_trains/add_merge_request_service_spec.rb

spec/features/merge_request/user_resolves_wip_mr_spec.rb

spec/features/merge_request/user_sees_merge_widget_spec.rb

spec/models/merge_request_spec.rb

spec/services/auto_merge/base_service_spec.rb

...rvices/merge_requests/merge_orchestration_service_spec.rb