[Docs] Updating `units of compute` to `compute minutes`

What does this MR do and why?

The renaming of CI Minutes and related terms to units of compute has been revisited and the renaming updated to compute minutes. This MR updates the Docs to reflect that.

Related to: "CI minutes" > "compute minutes" (gitlab-com&2150 - closed)
Related to: [Docs] "CI minutes" > "compute minutes" (#407063 - closed)

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