Mempool metrics: "prechecked" renamed to "validated"

tezos/tezos!9137 (merged) introduces a breaking change: the octez_mempool_pending_prechecked metric is renamed to octez_mempool_pending_validated. Grafazos should be updated accordingly. Moreover, the legend for this metric should be changed from "Applied" to "Validated".

Note: tezos/tezos!9137 (merged) also removes octez_mempool_pending_applied, but this metric was already no longer used by Grafazos.

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