Updated releaseWizard.py to use timezone-aware objects to represent datetimes in UTC

[shubhamsrkdev]

Quoting from python release doc :

datetime: datetime.datetime’s utcnow() and utcfromtimestamp() are deprecated and will be removed in a future version. Instead, use timezone-aware objects to represent datetimes in UTC: respectively, call now() and fromtimestamp() with the tz parameter set to datetime.UTC.

Owing to this made two types of changes:

1. Replaced datetime.utcnow() with datetime.now(tz=timezone.utc)
2. Replaced datetime.utcfromtimestamp(timestamp1) with datetime.fromtimestamp(timestamp=timestamp1, tz=timezone.utc)

Fixes #14063

[shubhamsrkdev]

@ChrisHegarty / @rmuir fyi please check this whenever convenient. TIA!