[Bug] Use UTC time zone when interpolating time variables, and not Locale server timezone

[ivanahuckova]

Ehen interpolating the time range for $__timeFrom and $__timeTo, instead of passing timeRange.From and timeRange.To, we should pass timeRange.From.UTC(). In the current implementation, the time zone is not explicitly specified, so it defaults to the Local time zone of the Grafana server, which can be incorrect and unexpected. This fix will at least ensure consistency in the time zone being used (other data sources handle this in the same way). Once this change is implemented, it would resolve the issue for cases where users have selected the UTC time zone.

[ivanahuckova]

@yesoreyeram I think this should be fine to implement, but will wait for you so we can discuss.

[marefr]

Some additional thoughts regarding variables and timezones https://github.com/grafana/support-escalations/issues/11969#issuecomment-2331365821