Refactor: replace make_simplified_union with UnionType.make_union (#8624)

[lsrafael13]

This PR replaces calls to make_simplified_union with the more modern UnionType.make_union across several files, as part of the cleanup and modernization discussed in issue #8624.

This change does not modify behavior but standardizes the way union types are created in the type system, improving code consistency.

Affected files:

• binder.py
• checker.py
• checkexpr.py
• checkmember.py
• checkpattern.py
• erasetype.py
• meet.py
• mypy/plugins/attrs.py
• mypy/plugins/ctypes.py
• mypy/plugins/default.py
• mypy/plugins/enums.py
• semanal_shared.py
• suggestions.py
• typeops.py

[sterliakov]

Okay, was this generated by some LLM? That's not the difference between UnionType.make_union and make_simplified_union - the former is a fast factory while the latter is a deep check that produces a minimal union (such that no item in the returned union is a subtype of some other item).

[lsrafael13]

I'm closing this PR after realizing that UnionType.make_union is not a safe drop-in replacement for make_simplified_union.
These two functions serve different purposes:
make_simplified_union simplifies unions by removing subtypes and redundancies.
UnionType.make_union is a fast constructor that skips simplification.
My initial intention was to help with the refactoring proposed in issue #8624, but I misunderstood the functional difference.
Thank you for the feedback — it helped me better understand the internals of mypy. I’ll be more careful in future contributions.