Backport of docs: Nomad 1.8 release notes into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #22104 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

The below text is copied from the body of the original PR.

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1. This is an attempt to replicate similar release notes documentation present in Consul to Nomad. This helps tremendously in terms of feature discovery and deprecation notices.
2. I didn't try to list every new feature in this release notes, just point out some key highlights.
3. This also changes some plugins to deprecated, and removes the rkt plugin from our docs.
4. Adds LTS and multiple vault and consul support to our Enterprise docs.

Preview: https://nomad-f713yjt7d-hashicorp.vercel.app/nomad/docs/release-notes/nomad/v1_8_x

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Overview of commits

• 6493bc6