Improve server_default / existing_server_default typing

[CaselIT]

The public typing have an incorrect type for it.
Use the correct one defined here:

alembic/alembic/ddl/base.py

Line 39 in ade17cf

_ServerDefault = Union["TextClause", "FetchedValue", "Function[Any]", str]

[Olegt0rr]

Wonder about default value False.

alembic/alembic/operations/ops.py

Lines 1836 to 1838 in 9d6e212

	existing_server_default: Optional[
	Union[str, bool, Identity, Computed]
	] = False,

and some other places - the same.

Changing it to None failed some tests with mssql generated sql 0_o

[CaselIT]

That's not the issue. False there has a particular meaning