Clean up schema for table push_rules

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Per 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 FK: project_id -> projects (26 entries violate this rule)
• Missing constraint: project_id NOT NULL (1 entry violates this rule)