Fix ActiveModel::UnknownAttributeError for `initialize_with_readme`

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/816241

Similar to Restrict `repository_object_format` update for ... (!153411 - merged)

Problem

optional_project_params_ce list is used for both create and update endpoints. However, initialize_with_readme field is only supported by create endpoints.

Solution

Move initialize_with_readme_format field to optional_create_project_params_ce group, so it won't be available for update endpoints.