Ensure ci_pipelines.iid set before transaction in commit status API

Prior to this we were using `create!` which triggered the
`ensure_project_iid!` in a callback. This causes a
`CrossDatabaseModificationAcrossUnsupportedTablesError`. To avoid this
we will create the `iid` before saving. This is consistent with all
other places we create pipelines.

This change still doesn't allow us to remove
`spec/requests/api/commit_statuses_spec.rb` from the allowlist as there
are other calls to `project.ci_pipelines.create!` in this file.

We need a more holistic solution for that which is being discussed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75316 .