Fix w.r.t. the removal of `eq_sorted_irr` in MathComp

[pi8027]

• Context: eq_sorted_irr has been deprecated and renamed to irr_sorted_eq in MathComp 1.12.0 (see math-comp/math-comp#646), and this deprecation alias will be removed in MathComp 1.13.0 (see math-comp/math-comp#727).
• This change adds a local alias for irr_sorted_eq (which serves as a compatibility layer for MathComp 1.11.0) in ordtype.v, deprecates and renames eq_sorted_ord to ord_sorted_eq so that it is compatible with the current naming convention of MathComp, and replaces the most occurrences of eq_sorted_irr with ord_sorted_eq.

[clayrat]

Thanks! Sorting out the deprecations introduced in latest SSReflect and Coq versions is on my to-do list, adding a compat layer is a nice trick!