Docs: Update configuration.md (login_maximum_inactive_lifetime_duration, login_maximum_lifetime_duration) (#31000)

Grafana [v7.2.0-beta2](https://github.com/grafana/grafana/releases/tag/v7.2.0-beta2) brought support for `login_maximum_inactive_lifetime_duration` and `login_maximum_lifetime_duration` to replace the older way of specifying the same flavor of timeouts (`login_maximum_inactive_lifetime_days` and `login_maximum_lifetime_days`). Unfortunately, this improvement did not make it to `configuration.md`.

The suggested changes reflect the current state of `conf/defaults.ini`.