Enable IPv6

[josecelano]

The current tracker configuration uses only IPv4.

It would be suitable for testing to:

1. Add more trackers using IPv6.
2. Or change the current one to use IPv6.

If we change them to IPv6, we should continue accepting IPv4 requests. See torrust/torrust-tracker#1368.

[josecelano]

It requires enabling beta features on Digital Ocean.