Dump the precision for datetime columns following the new defaults

[aidanharan]

The CI is failing following the release of Rails 7.0.2, see https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/runs/5123050136?check_suite_focus=true:

SchemaDumperTestSQLServer#test_0002_sst_datatypes_migration [/activerecord-sqlserver-adapter/test/cases/schema_dumper_test_sqlserver.rb:82]:
Precision of 6 not found in:
t.datetime "datetime_col".
Expected: 6
  Actual: nil

The test is failing following rails/rails#44358. When dumping a datetime column that uses the default precision of 6, the column will now be dumped with precision nil rather than 6.