node drain default behavior for -ignore-system

In https://github.com/hashicorp/nomad/pull/8606#discussion_r467480257 @jippi raised the question of whether `nomad node drain` should have the [`-ignore-system`](https://www.nomadproject.io/docs/commands/node/drain#ignore-system) flag set by default.

Currently the default behavior is that service and batch jobs are drained, and then system jobs are drained. However, internal allocation runner post-run hooks (ex. deregistering from Consul, cleaning up disk) don't block draining the system jobs, so they may run concurrently with the system jobs being drained. With the `-ignore-system` flag, the system jobs are never drained, which makes them available during shutdown.

The `-ignore-system` flag appears to be strictly more useful than not having it set. Some example scenarios where you'd want to have a system job that runs until all other workloads have completed:
* log shippers
* monitoring agents
* ingress proxies (ex. Nginx or HAProxy in front of web services)
* CSI node plugins

Changing the default behavior would break backwards compatibility, so we want to solicit feedback from the community about whether this change would be disruptive.

Start with the `nomad node drain` command and its `-ignore-system` documentation, then review the linked pull request discussion and the documented current behavior. Done means reaching and recording a community-supported decision about changing the default, including the backward-compatibility impact.

Written by the indexing model from the issue text.