Modify the DateField Field plugin so it's used for D6 & D7

Here's a first patch.

+++ b/core/modules/datetime/src/Plugin/migrate/field/d7/DateField.php
@@ -0,0 +1,22 @@
+ *   type_map = {
+ *     "date" = "datetime",
+ *     "datestamp" = "datetime",
+ *     "datetime" = "datetime",
+ *   },

I created a dedicated (empty) D7 plugin because this type_map is what I found in the d7_field migration template static_map and it's different from the D6 DateField plugin type_map:

 *   type_map = {
 *     "date" = "datetime",
 *     "datestamp" = "timestamp",
 *     "datetime" = "datetime",
 *   }

Maybe we want the same thing as D6 since the difference is in the destination field type, not in the source. But then we would have to fix some tests that will fail because datetime is expected instead of timestamp.

Here's a new patch using the same plugin as D6 and with the tests updated.

Should we move the DateField plugin out of the d6 namespace now that it's used for D7 too?

Updated IS & title.

I copied the DateField class out of the d6 namespace and deprecated the old class.
Same for the Unit test.