all: make timestamp-based fork checks based on uint64 (#26474)

This PR changes the API so that uint64 is used for fork timestamps.
It's a good choice because types.Header also uses uint64 for time.

Co-authored-by: Felix Lange <fjl@twurst.com>

core/txpool/txpool.go

@@ -1312,7 +1312,7 @@ func (pool *TxPool) reset(oldHead, newHead *types.Header) {
 	pool.istanbul = pool.chainconfig.IsIstanbul(next)
 	pool.eip2718 = pool.chainconfig.IsBerlin(next)
 	pool.eip1559 = pool.chainconfig.IsLondon(next)
-	pool.shanghai = pool.chainconfig.IsShanghai(big.NewInt(time.Now().Unix()))
+	pool.shanghai = pool.chainconfig.IsShanghai(uint64(time.Now().Unix()))
 }
 
 // promoteExecutables moves transactions that have become processable from the