Consistently use type-abstract error code

[ilevkivskyi]

Ref #4717

Although function use case is much more important, the variable assignment should use the same error code, otherwise this may cause confusion.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/base/__init__.py:381: error: Can only assign concrete classes to a variable of type "Type[DatabaseTable]"  [misc]
+ ibis/backends/base/__init__.py:381: error: Can only assign concrete classes to a variable of type "Type[DatabaseTable]"  [type-abstract]

[JukkaL]

Thanks! We could later discuss turning off this error code by default, since this seems to cause false positives, right?