workflows: omit stopped workflows from cumulative state totals

[oliver-sanders]

• Closes gscan: omit stopped workflows from cumulative state totals #1930
• Have purposefully left the "latest state tasks" logic unchanged as we are planning to remove this imminently.

It doesn't look like there are any tests for this logic at present. I haven't added any in this PR as yet.

There's ongoing refactor work here, so might be something to pick up on the other side?

Check List

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• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
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• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.