Add foreign key on `project_group_links.group_id` & remove invalid data

What does this MR do and why?

For #382397 (closed)

See https://gitlab.com/gitlab-com/dev-sub-department/section-dev-request-for-help/-/issues/26#note_1169262972 for background.

This MR does 2 things:

• Adds foreign key on project_group_links.group_id linked to group.id
• Existing project_group_links records, that point to a non existent group_id are deleted via a background migration.

On gitlab.com, as of 3rd Apr, 2023, there is only 1 such record -https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/17444/commands/58383

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