Resolve "Introduce round-robin project creation to spread load over multiple shards"

Allow multiple shards to be enabled in the admin settings page, balancing project creation across all enabled shards.

* `f.select ..., multiple: true` isn't the most beautiful UI in the world, but switching to `collection_check_boxes` (or a facsimile thereof) isn't trivial
* Should `pick_repository_storage` be a method of `ApplicationSetting`, or `Project`? It's going to accrete logic over time so perhaps it should be its own class already?
* This is written to avoid the need for a database migration, so it is`serialize :repository_storage` without `, Array`. This is tested, but alternatives include:
  * Add a database migration
  * Write a custom Coder that will accept a String or Array in `load` and always `dump an Array.

Closes #24059

See merge request !7273
Signed-off-by: Rémy Coutable <remy@rymai.me>