Move priority/backlog to global label definition

[dhiller]

The label priority/backlog should be moved to the global section .default.labels in the label config file labels.yaml - adjacent to the label priority/critical-urgent

Editing our GitHub labels is done by editing the labels.yaml - similarly to adding a label 2, and running make update-labels afterwards, which will update the labels.md file.

Please do all this in one commit, so we have the changes grouped together nicely.

See issue comment as example.

[dhiller]

/kind tracker
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[kubevirt-bot]

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