docker: remove email auth config parameter

[mismithhisler]

Description

The email auth parameter was deprecated and is being removed in a future release of docker.

Testing & Reproduction steps

Links

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
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad product documentation, which is stored in the
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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.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
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• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[gulducat]

I like to err on the side of caution with these, but good golly, it was removed a looong time ago. I found this https://github.com/moby/moby/releases/tag/v1.11.0 (ctrl+f "email") dated 2016(!)

If you have any other links to reference, that would be welcome, but otherwise, LGTM!

[gulducat]

Oh, do be sure to update the docs, too. I think this may be the only spot?

• https://developer.hashicorp.com/nomad/docs/job-declare/task-driver/docker#email