mod: Update Nomad Go module to github.com/hashicorp/nomad/v2

[jrasell]

The last Nomad release was tagged with v2.0.0. While Nomad's core
is not designed to be directly imported, it happens. Go's module
constraints requires a module with a tagged 2.x to have a module
suffix of /v2; the same for 3.x and such.

This change modifies the Nomad module to have the v2 suffix and
updates all imports as such.

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
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• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
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  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

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