feat: change description

Author: mcrakhman

.changelog/unreleased/breaking-changes/2966-mempool-failed-tx-code-label.md

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+- [mempool] \#2966 Add a `code` label to the `cometbft_mempool_failed_txs`
+  Prometheus counter so failed-tx counts can be broken down by ABCI response
+  code, with `precheck` / `postcheck` sentinels for node-level filter
+  rejections. Also demote the per-tx "failed tx" logs added in #2965 from
+  Error to Debug to keep them out of the default-level log stream.