[8.x] Support useCurrentOnUpdate for MySQL datetime column types

[timmartin19]

This commit introduces support for useCurrentOnUpdate for MySQL datetime
column types. MySQL has supported using ON UPDATE CURRENT_TIMESTAMP
for DATETIME columns since v5.6.5 (and Laravel supports MySQL 5.7+
from what I can tell). Laravel previously only supported useCurrentOnUpdate
for TIMESTAMP columns in MySQL.

This allows using DATETIME columns for metadata like created_at
and updated_at more effectively. Since TIMESTAMP columns in
MySQL suffer from the Year 2038 Problem,
using DATETIME columns is preferred in some applications.

We set the fractional seconds precision to be consistent
throughout the column initialization (for the column type, the default,
and the on update). MySQL requires that the precision be identical
throughout. Note that this implicitly fixed a (likely rare) bug when
users tried to run something like ->dateTime('foo', 2)->useCurrent().
Previously this would result in MySQL error code 1067 for invalid
default values. It will not set the default to have the same precision
as the column definition.

The actual code was borrowed heavily from typeTimeStamp. Since they
are distinct column types, it felt incorrect to DRY it up despite their
similarities.

Relevant MySQL documentation:
MySQL 5.6
MySQL 5.7
MySQL 8.0

[timmartin19]

Note that this is my first PR in any of the Laravel repos, so please let me know if there's anything I can improve on going forward!