Warn during job plan of task->resources->network stanza usage deprecation

[nickethier]

In 0.12 the usage of a network stanza inside of a task's resource stanza was deprecated as new features have only been introduced via the group network stanza. This should be warned on job submission.

[Legogris]

Can the release notes also be amended to mention this? https://github.com/hashicorp/nomad/releases/tag/v0.12.0

There's also several places in docs and official tutorials where task-resource-level networking is utilized, shouldn't these be addressed before technical deprecation?

[github-actions]

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