Add containing_date to List Group milestones REST API

Proposal

While reviewing https://gitlab.com/gitlab-org/security-products/analyzers/sast-analyzer-deps-bot/-/merge_requests/26+s, I noticed that the Group.milestones GraphQL endpoint provides a containingDate parameter, while the List group milestones REST API endpoint does not.

The purpose of this issue is to add a containing_date parameter to the List group milestones REST API endpoint for parity with the Group.milestones GraphQL endpoint. This will allow us to simplify code such as the following since we can have the API perform the time filtering instead of needing to filter it on the client side.