When the `client.servers` block is parsed, we split the port from the
address. This does not correctly handle IPv6 addresses when they are in URL
format (wrapped in brackets), which we require to disambiguate the port and
address.
Fix the parser to correctly split out the port and handle a missing port value
for IPv6. Update the documentation to make the URL format requirement clear.
Fixes: https://github.com/hashicorp/nomad/issues/20310
