fix: serde rename camel case for execution payload body #16810

Job	Run time
Check for 'skip-ci' label	1s
check-code	8m 12s
target-branch-check	1s
check-msrv	4m 30s
op-pool-tests	19m 1s
network-tests	14m 38s
release-tests-ubuntu	14m 5s
beacon-chain-tests	3m 50s
release-tests-windows	20m 14s
cargo-udeps	6m 33s
basic-simulator-ubuntu	20m 1s
cli-check	15m 30s
compile-with-beta-compiler	13m 35s
debug-tests-ubuntu	6m 3s
dockerfile-ubuntu	14m 28s
doppelganger-protection-test	20m 2s
ef-tests-ubuntu	6m 43s
execution-engine-integration-ubuntu	10m 37s
fallback-simulator-ubuntu	17m 48s
slasher-tests	2m 24s
state-transition-vectors-ubuntu	9m 8s
test-suite-success	0s
	3h 47m 24s

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