Add member namespace id db constraints

What does this MR do and why?

Describe in detail what your merge request does and why.

#349640 (closed)

• add NOT NULL constraint for member_namespace_id column
• change add_concurrent_foreign_key :members, :namespaces, column: :namespace_id, on_delete: :nullify to add_concurrent_foreign_key :members, :namespaces, column: :namespace_id, on_delete: :cascade

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#377433 (comment 1139215782)

• Migration file 1: Re-add the check constraint (not valid)
• Migration file 2: Validate the check constraint
• Migration file 3: Validate the new FK (fk_2f85abf8f1)
• Migration file 4: Drop the old FK with the SET NULL clause (fk_aa82dcc1c6)

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