Merge spec 1.1.6

[g11tech]

main tracker: #3410

• Cleanup/Devnet tasks
  • coinbase to fee_recipient, fee_recipient to suggested_fee_recipient, receipt_root to receipts_root #3492
  • is_merge -> is_merge_transition
  • Genesis execution block handling
• CL spec 1.1.6: proposer boost #3506
  • Remove difficulty from PowBlock (UPDATE: specs 1.1.5 specs on spec-runner fail on this and 1.1.6 spec can be updated once proposer boosting is implemented)
  • Update Forkchoice : 1. proposer boosting 2. atomic update to justified
• [ ] Add missing bellatrix tests to spec runner #3541 Independently tracked