core/txpool: demote error log to warn (ethereum#31332)

This error log in `legacypool.go` isn't necessary, since even though the
behavior is unexpected, it is handled correctly. A discussion on issue
ethereum#22301 concluded that this should instead be a warning log.

Author: alarso16

core/txpool/legacypool/legacypool.go

@@ -1604,7 +1604,7 @@ func (pool *LegacyPool) demoteUnexecutables() {
 			gapped := list.Cap(0)
 			for _, tx := range gapped {
 				hash := tx.Hash()
-				log.Error("Demoting invalidated transaction", "hash", hash)
+				log.Warn("Demoting invalidated transaction", "hash", hash)
 
 				// Internal shuffle shouldn't touch the lookup set.
 				pool.enqueueTx(hash, tx, false)