Set a lower priority fee for light client update

[alxiong]

Currently we are setting a 3 Gwei priority fee on top of the base fee (e.g. this tx), while in the same block there are more frugal tx that still got included with 0 priority fee (like this)

Since we don't care about strict or fast update on light client contract, we should be save money by setting a low or zero priority fee.

Other resources:

• etherscan's gas oracle (e.g. API response)
• EIP 1559 variables and definitions here

we will need to test this out in testnet first before deploying anything to production.