profiler: change the `polling interval` field from an integer to an ISO8601 interval

[oliver-sanders]

Follow on to #6663

(please do before 8.7.0 release or it'll become a functional config change)

In Cylc, we configure intervals in ISO8601 syntax, e.g, execution time limit = PT30S.

However, the poller has used an int, polling interval = 30.

Best to standardise on ISO8601 and use the isodatetime interface to convert that to an int.