Backport of ui: Fill service check background object for pending checks. into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24818 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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Fills the service check marker with a grey when in pending state, whereas it was previously empty. I personally found the empty space a little confusing at first glance, whereas having this addition makes it obvious what is happening with the check and that you can hover and see the status.

I chose the colour based on a quick look, trying to find a grey that wasn't too dark, but I don't really know what I am doing, so please let me know what changes to make.

I think it makes sense to backport. Let me know if you think I should add a changelog entry.

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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 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.

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Overview of commits

• 75d0ac6