refactor(RSSHandlerRoutine): switch to Instant

[christolis]

From a recent conversation:

[13:09]Zabuzard: while at it, ZonedDateTime should be replaced by Instant in that file
[13:10]Zabuzard: does someone want to tackle that quickly or else we need to make a github issue 🙂
[13:56]christolis: what makes this change worthwhile?
[14:01]Zabuzard: its just the wrong class for the job
[14:01]Zabuzard: there is no reason to use ZDT in this case
[14:01]Zabuzard: Instant is the correct class
[14:02]Zabuzard: shouldnt require any deeper changes in this case, just a quick replacement should work```

[tj-wazei]

Looks reasonable to me.