remove mentions of deprecated Nomad versions in docs

[sofixa]

Description

A few docs had mentions such as "Starting Nomad x.x" where x.x are long since out of support, or contained obsolete upgrade guidance (e.g. to Raft version 3).

Contributor Checklist

• [ ] Documentation If the change impacts user-facing functionality such as the CLI, API, UI, and job configuration, please update the Nomad website documentation to reflect this. Refer to the website README for docs guidelines. Please also consider whether the change requires notes within the upgrade guide.

Reviewer Checklist

• [ ] Backport Labels Please add the correct backport labels as described by the internal backporting document.
• [ ] Commit Type Ensure the correct merge method is selected which should be "squash and merge" in the majority of situations. The main exceptions are long-lived feature branches or merges where history should be preserved.
• [ ] Enterprise PRs If this is an enterprise only PR, please add any required changelog entry within the public repository.
• [ ] Review that the deprecated configs/features for which the docs were removed have the corresponding code removed (e.g. client options for plugin config, or port_map for port mapping).

[aimeeu]

@sofixa Adrian - Is this PR still relevant? It's been open for quite a long time. The docs content has migrated to the web-unified-docs repo, so the updates in this PR need to be recreated in the other repo. I can do the docs update or help.

[tgross]

Is this PR still relevant?

This is still relevant because we have references to old versions floating around, but this PR as-is had some problems where we were removing text for configuration that's deprecated but not actually removed (and not scheduled to be removed, unfortunately).