chore(*): add mathlib4 synchronization comments (#18630)
Regenerated from the [port status wiki page](https://github.com/leanprover-community/mathlib/wiki/mathlib4-port-status).
Relates to the following files:
* `algebra.category.Mon.basic`
* `algebra.squarefree`
* `category_theory.closed.monoidal`
* `category_theory.single_obj`
* `data.zmod.coprime`
* `field_theory.perfect_closure`
* `linear_algebra.invariant_basis_number`
* `ring_theory.euclidean_domain`
* `ring_theory.flat`
* `ring_theory.int.basic`
* `ring_theory.principal_ideal_domain`
* `ring_theory.unique_factorization_domain`
