Gateway API: Recommend the `config` approach rather than `extraArgs`

[maelvls]

The config approach is much more convenient than to have to concatenate flags in the extraFlags field. The config approach isn't immediately obvious to users, as seen in https://github.com/cert-manager/cert-manager/pull/7121 where we found that there was no need for adding an extra helm value: one could simply use config.enableGatewayAPI.