`DeferredNow::format_rfc3339` uses wrong timezone information

[hasezoey]

Currently the timezone is defined with a Z but given a local time, but this will actually result in the wrong time https://github.com/emabee/flexi_logger/blob/b2447d08550b298a2f356e4ac725e402733f0484/src/deferred_now.rs#L52

for example in my current timezone which is Europe/Berlin (+1h) the following happens:

name	time
actual UTC time	2023-01-14T16:16:34.000Z
current function output	2023-01-14T17:16:34.000Z
using %Z	2023-01-14T17:16:34.000+01:00
using %z	2023-01-14T17:16:34.000+0100
using just %+	2023-01-14T17:16:34.000000000+01:00
for reference JS .toISOString would output*1	2023-01-14T16:16:34.000Z

*1 given the time of 2023-01-14T17:16:34.000+01:00 (from using %Z)

possible fixes:

• replace Z with %Z
• actually give the format functions a UTC time