MDEV-41344 Timestamp conversion does not work for TRADITIONAL mode

[midenok]

Field_temporal::get_copy_func() returns do_field_datetime whenever the session's sql_mode has NO_ZERO_DATE or NO_ZERO_IN_DATE (e.g. under sql_mode=TRADITIONAL), or when the two fields are not eq_def(), regardless of whether the field is a system-versioned row_end.

get_copy_func() checked for that value and returned early, before ever reaching the VERS_ROW_END check that installs
do_field_versioned_timestamp. So an ALTER TABLE .. FORCE meant to convert an old-format row_end silently copied the value unchanged instead: the conversion check correctly demanded a copy, but the copy step never applied it, with no error or warning.

The fix checks the row_end conversion need first, independently of what Field_temporal::get_copy_func() picked. row_end is server-maintained and never zero, so NO_ZERO_DATE does not apply to it.

Tested by "traditional" combination in old_timestamp.test, the test restores real pre-11.5 row_end fixtures and runs mariadb-upgrade --force.

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