"Ethereum Mainnet" Sync status label naming

[jimmygchen]

Describe the bug
On the top of the main dashbaord, there are two sync statuses being displayed: "Ethereum Mainnet" and "Beacon Chain".

@rickimoore confirmed that these are ${beaconUrl}/lighthouse/eth1/syncing (execution engine sync status), ${beaconUrl}/eth/v1/node/syncing (beacon chain sync status). Would be good to rename the "Ethereum Mainnet" label to something like "Execution Engine" to avoid confusion.

Screenshots