Backport of ui: show region in header gutter when only one region exists into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24320 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

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

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This PR adds a plain text label of the region to the header when there is only one region present. Before, nothing was showin in this case, and a dropdown was shown on federated clusters.

The use case here is for operators of multiple non-federated Nomad clusters, when all the UI's involved otherwise look identical.

Fixes #14893

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Additionally, resolves https://github.com/hashicorp/nomad/issues/24337 and adds activeRegion / region switcher dropdown information upon signing in.

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Overview of commits

• 4dbcdd103ad39f8e4b5b6d772f8d25928ddc8bb6