core/txpool/legacypool: use uint256.Int instead of big.Int (#28606)

This change makes the legacy transaction pool use of `uint256.Int` instead of `big.Int`. The changes are made primarily only on the internal functions of legacypool. 

---------

Co-authored-by: Martin Holst Swende <martin@swende.se>

eth/backend.go

@@ -229,7 +229,7 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 	}
 	legacyPool := legacypool.New(config.TxPool, eth.blockchain)
 
-	eth.txPool, err = txpool.New(new(big.Int).SetUint64(config.TxPool.PriceLimit), eth.blockchain, []txpool.SubPool{legacyPool, blobPool})
+	eth.txPool, err = txpool.New(config.TxPool.PriceLimit, eth.blockchain, []txpool.SubPool{legacyPool, blobPool})
 	if err != nil {
 		return nil, err
 	}

eth/protocols/eth/handler_test.go

@@ -117,7 +117,7 @@ func newTestBackendWithGenerator(blocks int, shanghai bool, generator func(int,
 	txconfig.Journal = "" // Don't litter the disk with test journals
 
 	pool := legacypool.New(txconfig, chain)
-	txpool, _ := txpool.New(new(big.Int).SetUint64(txconfig.PriceLimit), chain, []txpool.SubPool{pool})
+	txpool, _ := txpool.New(txconfig.PriceLimit, chain, []txpool.SubPool{pool})
 
 	return &testBackend{
 		db:     db,