Use UTC (Z) for timestamps to avoid ambiguity

[keighrim]

Currently, mmif-python outputs timestamps in local time without specifying a timezone offset. This can lead to ambiguity, especially when mmif files are processed or generated within Docker containers where system timezone data might be missing, inconsistent, or set to UTC by default, differing from the host's or developer's local time.

This feature request proposes standardizing the timestamp output to always use UTC, indicated by the 'Z' (Zulu time) suffix, to ensure clarity and consistency across different environments.