feat(nodebuilder/p2p): Add mainnet genesis hash (celestiaorg#2889)

![IMG_A965E35247A9-1](https://github.com/celestiaorg/celestia-node/assets/41963722/f2ff92df-567f-44fe-9378-0179aa14df6f)

Author: renaynay

nodebuilder/p2p/genesis.go

@@ -23,7 +23,7 @@ func GenesisFor(net Network) (string, error) {
 
 // NOTE: Every time we add a new long-running network, its genesis hash has to be added here.
 var genesisList = map[Network]string{
-	Mainnet: "",
+	Mainnet: "6BE39EFD10BA412A9DB5288488303F5DD32CF386707A5BEF33617F4C43301872",
 	Arabica: "5904E55478BA4B3002EE885621E007A2A6A2399662841912219AECD5D5CBE393",
 	Mocha:   "B93BBE20A0FBFDF955811B6420F8433904664D45DB4BF51022BE4200C1A1680D",
 	Private: "",