PYTHON-4885 Fix legacy extended JSON encoding of DatetimeMS

[ShaneHarvey]

PYTHON-4885 Fix legacy extended JSON encoding of DatetimeMS

Note I opted not to introduce logic to parse previously encoded invalid "$date" data for a few reasons:

1. It's not clear if that behavior would be useful. No one has reported this as an issue.
2. We can always introduce it later if we get new info that it's required.
3. Adding it is problematic since it's invalid. We don't know if the data is genuinly invalid or a result of the encoding bug. For example, does the user intend this '{"dt": { "$date" : "1970"}}' to be the year 1970 or is it 1970 milliseconds after the epoch?

Old behavior:

>>> dumps({'d': DatetimeMS(1)}, json_options=LEGACY_JSON_OPTIONS)
'{"d": {"$date": "1"}}'

New behavior:

>>> dumps({'d': DatetimeMS(1)}, json_options=LEGACY_JSON_OPTIONS)
'{"d": {"$date": 1}}'

This PR also fixes a bug in the parsing logic where we would incorrectly raise an IndexError on invalid $date:

Old behavior:

>>> loads('{"dt": { "$date" : "1"}}')
Traceback (most recent call last):
...
  File "/Users/shane/git/mongo-python-driver/bson/json_util.py", line 624, in _parse_canonical_datetime
    elif dtm[-6] in ("+", "-") and dtm[-3] == ":":
         ~~~^^^^
IndexError: string index out of range

New behavior:

>>> loads('{"dt": { "$date" : "1"}}')
Traceback (most recent call last):
  File "/Users/shane/git/mongo-python-driver/bson/json_util.py", line 625, in _parse_canonical_datetime
    elif dtm[-6] in ("+", "-") and dtm[-3] == ":":
         ~~~^^^^
IndexError: string index out of range

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
...
  File "/Users/shane/git/mongo-python-driver/bson/json_util.py", line 641, in _parse_canonical_datetime
    raise ValueError(f"time data {dtm!r} does not match ISO-8601 datetime format") from exc
ValueError: time data '1' does not match ISO-8601 datetime format