[Shanghai] Rename DIFFICULTY to PRERANDAO

[ChihChengLiang]

Task to do

The main goal is to rename the opcode DIFFICULTY to PRERANDAO to support the Shanghai update.

Some details:

• update all references of DIFFICULTY in blocktable. Check places like pi_circuit.py, tables.py, BlockContextFieldTag, and tables.md.
• update the opcode in places like opcode.py and 41COINBASE_45GASLIMIT_48BASEFEE.md
• Everywhere mentioned difficulty should be renamed.

References

• https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/paris.md
• https://eips.ethereum.org/EIPS/eip-4399
• Circuit issue privacy-scaling-explorations/zkevm-circuits#1376