allow converting from DateTime2 to Datetimen in 'tds73'

[Geo-W]

This pr tries to address the following problem:
When inserting NaiveDateTime into ms datetime columns with bulk_insert and tds73 is turned on by default, it would raise
Err` value: BulkInput("invalid data type, expecting Some(VarLenSized(VarLenContext { type: Datetimen, len: 8, collation: None })) but found DateTime2(Some(DateTime2 { date: Date(693604), time: Time { increments: 330110000000, scale: 7 } }))

cannot just disable tds73 feature as date/time might in the same table with datetime simultaneously and into_sql/to_sql is impl for date/time in without tds73 only.

normal inserting with execute does not get this issue as it calls DateTime2 to None when converting.

tiberius/src/tds/codec/column_data.rs

Lines 615 to 619 in 98943b2

	(ColumnData::DateTime2(Some(dt)), None) => {
	let len = dt.time().len()? + 3;
	dst.extend_from_slice(&[VarLenType::Datetime2 as u8, dt.time().scale(), len]);
	dt.encode(&mut *dst)?;
	}

Don't know whether it is a good solution. Or maybe any other impl can solve this issue is appreciated.
Thank you so much~