mgr/dashboard: Fix timestamps in APIs

mgr/dashboard: Fix timestamps in APIs

- remove 'Z' from rbd APIs which are returning now `aware` timestamp
- `datetime.utcfromtimestamp` is deprectated so using  `datetime.fromtimestamp(timestamp, tz=tz=timezone.utc)` thereby returning only `aware` timestamp and removing 'Z'.
- similarly `datetime.utcnow()` is deprecated , migrated to `datetime.now(timezone.utc)`

https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow
https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp

Signed-off-by: Afreen Misbah <afreen@ibm.com>