CLN: unify error message when parsing datetimes with mixed time zones with utc=False
xref #57275, #55793

unified ValueError message when parsing datetimes with mixed time zones with utc=False
so far we get two different ValueError messages:
```
import pandas as pd
from datetime import datetime
import pytz

str_my ="2020-10-26 00:00:00+06:00"
ts = pd.Timestamp("2018-01-01", tz="US/Pacific")
dt = datetime(2020, 1, 1, 18, tzinfo=pytz.timezone("Australia/Melbourne"))

dt1 = pd.to_datetime([ts, str_my])
# ValueError: Mixed timezones detected. pass utc=True in to_datetime or tz='UTC' in DatetimeIndex to convert to a common timezone.
dt2 = pd.to_datetime([str_my, ts])
# ValueError: cannot parse datetimes with mixed time zones unless `utc=True`

dt1 = pd.to_datetime([dt, str_my], utc=False)
# ValueError: Mixed timezones detected. pass utc=True in to_datetime or tz='UTC' in DatetimeIndex to convert to a common timezone.
dt2 = pd.to_datetime([str_my, dt], utc=False)
# ValueError: cannot parse datetimes with mixed time zones unless `utc=True`
```
Thanks @natmokval 