For now, timestamp magic columns are only allowed for real physical
columns, it is not a matter for newly created app, but it is harder to
get the usefulness for legacy databases.

The reason that doesn't work is some low-level API does not care about
attribute aliases. I think that uses low-level API without attribute
alias resolution for timestamp attributes is not intended (e.g.
`updated_at_before_type_cast` works, but `has_attribute?("updated_at")`
and `_read_attribute("updated_at")` doesn't work).

I've addressed all missing attribute alias resolution for timestamp
attributes to work that consistently.

Fixes #37554.