Fix deprecation warning for datetime.utcnow()

[mks-h]

Fixes the following warning:

/usr/local/bin/termux-apt-repo:137: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  print("Date: " + datetime.datetime.utcnow().strftime('%a, %d %b %Y %H:%M:%S UTC'), file=release_file)