Updates `available_until` regex to allow timezones

[richfab]

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MobilityData created this Pull Request on behalf of @hbruch.

What problem does your proposal solve? Please begin with the relevant issue number. If there is no existing issue, please also describe alternative solutions you have considered.

Fixes https://github.com/MobilityData/gbfs-json-schema/issues/95

available_until is a Datetime which allows timezones but the regex does not accept timezones.

GBFS spec definition:

Datetime (added in v2.3)- Combination of a date and a time following ISO 8601 notation. Attributes : year, month, day, hour, minute, second, and timezone.

What is the proposal?

Update available_until regex to allow timezones.

Before	After

Which files are affected by this change?

v2.3/free_bike_status.json and v3.0-RC/vehicle_status.json