Switched Milestone and Release to many-to-many relationships

What does this MR do?

Setting up a many-to-many relationship at the Rails level between Milestone and Release.

Original CE merge request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32656

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/66986