Change release evidence creation status API from `202 Accepted` to `201 Created`

Spawned from the discussion here #211675 (comment 338189226)

Could we consider adding the id into the empty Evidence array, and then correcting the status code as a breaking change in 14.0?

Currently, we return 202 Accepted on POST releases/:tag/evidence, not 201 Created.

Maybe we decided to do so because creation actually happens in the background job and it was logical. 🤷

But after #214819 it might be more logical to return 201.