cmd: set DefaultGasLimit to 30M (ethereum#24680)

* cmd: set DefaultGasLimit to 30M, rem deprec. Flag

* cmd: revert flag deprecation

Author: MariusVanDerWijden

eth/ethconfig/config.go

@@ -84,7 +84,7 @@ var Defaults = Config{
 	TrieTimeout:             60 * time.Minute,
 	SnapshotCache:           102,
 	Miner: miner.Config{
-		GasCeil:  8000000,
+		GasCeil:  30000000,
 		GasPrice: big.NewInt(params.GWei),
 		Recommit: 3 * time.Second,
 	},