Feature: Default `--port` and `--port6` to be identical

[yorickdowne]

Description

Currently, the --port and --port6 options have distinct default values

This means additional configuration steps for users, whether for ufw or host-mapped docker ports.

Feature suggestion

From a networking perspective, the ports could be the same and a program could listen on v4 and v6. This is the standard behavior for dual-stack applications.

This does work for Lighthouse when configured that way: I am suggesting to make it the default.

[chong-he]

Both --port and --port6 now defaults to 9000 by default with #6796