Design UI for creating a repository as a sha256 repo

In order to support creating projects with sha256 as the hashing algorithm through the user interface, we first need a design for how that will look.

Based on the recommendation #426803 (comment 1644361155) that we make this make it very hidden and/or be extremely clear that this might break core functionality:

• label this feature as an experiment (https://design.gitlab.com/usability/feature-management#highlighting-feature-versions). As a experiment we make it known that it is unstable, can cause data loss, and can be removed at any time.
  • The popover needs to be here to explain what an experiment is.
• To make this checkbox more hidden from the current project creation form, we will put it under an accordion.

The help text for this checkbox currently reads as:

Might break existing functionality with other repositories or APIs. It's not possible to change SHA-256 repositories back to the default SHA-1 hashing algorithm.

Accordion	Accordion - expanded