Incorrect modal warning on study ad form

## Description

When saving the Study Ad form, there is a confusing/incorrect message about the experiment runner. The warning asks the researcher to confirm that they want to save changes, with this message:

> You have already built an experiment runner! Because you are changing the runner version, you will need to rebuild these before you can preview or start your study.

However, the experiment runner version info is not part of the Study Ad form anymore, so there is no way the runner version has been changed.

## Proposed solution

This message should either:
1. be removed in favor of the red text at the bottom of the Study Design page ("Changes to any of these values (aside from code formatting), may cause your study to be rejected or need to be built again.") or
2. be moved to the Study Design page in place of the red text at the bottom

Here's the relevant part of the `study_edit.html` template:

```
{% if study.built or save_confirmation %}

Start with the study_edit.html template and inspect how the Study Ad form and Study Design page render their save warnings. Compare the existing invalidate-build-warning with the red text described in the issue, then determine which warning belongs on each page. Done means the incorrect Study Ad modal warning is removed or relocated consistently, with the resulting UI behavior checked manually.

Written by the indexing model from the issue text.