utcfromtimestamp deprecated

https://github.com/MITHaystack/srt-py/blob/master/srt/dashboard/layouts/graphs.py#L266 is deprecated since Python 3.12.

It can be repleced by `fromtimestamp`:
```
from datetime import datetime, timezone

ts = 1571595618.0
x = datetime.utcfromtimestamp(ts)
print(x)

y = datetime.fromtimestamp(ts, tz=timezone.utc)
print(y)
```

> 2019-10-20 18:20:18
> 2019-10-20 18:20:18+00:00