Add group releases to GraphQL

We already have GroupReleasesFinder, which is used for internal Groups::ReleasesController.

Groups::ReleasesController is being used in the Merge Request filter on the group level

Ideally, we want to migrate this to the API. But there's no API for group releases yet. Since for this UI filter we actually only needs tag names, GraphQL API would be ideal.

Implementation guide

• Add GroupReleasesResolver similar to ReleasesResolver, but use GroupReleasesFinder
• Add releases to the GroupType
• Add tests for GroupReleasesResolver
• Add tests similar to https://gitlab.com/gitlab-org/gitlab/-/blob/a1c9fc6864b643ae8b93df25ccdb18dbc34313e4/spec/requests/api/graphql/project/releases_spec.rb#L5-L5