Change BUILDER_WITHDRAWAL_PREFIX from 0x03 to 0xB0

Follows ethereum/consensus-specs#5416, which changed the builder
withdrawal credential prefix to 0xB0 (lands in v1.7.0-alpha.12).

Author: barnabasbusa

consensoor/spec/constants.py

@@ -449,7 +449,7 @@ def MAX_BUILDERS_PER_WITHDRAWALS_SWEEP() -> int:
 DOMAIN_PROPOSER_PREFERENCES: Final[bytes] = b"\x0d\x00\x00\x00"
 DOMAIN_BUILDER_DEPOSIT: Final[bytes] = b"\x0e\x00\x00\x00"
 
-BUILDER_WITHDRAWAL_PREFIX: Final[bytes] = b"\x03"
+BUILDER_WITHDRAWAL_PREFIX: Final[bytes] = b"\xb0"
 
 
 # Gloas churn limits (EIP-8061): split exit and activation churn limits