Change references to `strftime("%Y-%m-%d")` to `.iso8601`

This is a refactor.

Throughout the GitLab codebase, we use strftime("%Y-%m-%d") on dates in many places. This can be replaced with a much more friendly iso8601 method.

If the object is a datetime, to_date will also need to be called.

time = Time.current
=> Wed, 10 May 2023 22:37:31.567040000 UTC +00:00

> time.strftime("%Y-%m-%d")
=> "2023-05-10"

> time.iso8601
=> "2023-05-10T22:37:31Z"

> time.to_date.iso8601
=> "2023-05-10"