Define sharding key for `snippets`

Problem

See epic description: &13636 (closed)

Context

At this point, the snippets migration should be completed.

Task

Set sharding_key to https://gitlab.com/gitlab-org/gitlab/blob/db7dd37347a156203b8368ae2e6a72e594687e43/db/docs/snippets.yml#L12.

sharding_key:
  project_id: projects
  organization_id: organizations