Show runs times in UTC

[gsnedders]

Having filed https://github.com/web-platform-tests/results-analysis/pull/194, @nt1m questioned "Is it 19/09 - 20/09 or 20/09 - 21/09 ?".

And, like, we're seeing different things on the dashboard:

@nt1m was seeing:

Whereas I was seeing:

It doesn't really make too much sense to be localising the run times, as this makes it confusing when trying to figure out what daily aligned runs are. wpt.fyi should instead always use UTC for all run times, everywhere.