Clean up schema for table ci_runner_projects

From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing constraint: project_id NOT NULL
• Missing FK: runner_id -> runners (2 entries violate this rule)
• Missing FK: project_id -> projects (178 entries violate this rule)