[Sharding] Fix the EFFECTIVE_BALANCE_MAX_DOWNWARD_DEVIATION constant calculation

[Nashatyrev]

Fix back to the original formula here: 68844f2

[Nashatyrev]

However the original formula looks not quite accurate

For example taking default spec constants:

Suppose the max blob gas cost is 9.5Eth,
EFFECTIVE_BALANCE_MAX_DOWNWARD_DEVIATION = 1Eth - 1Eth * 1 / 4 = 0.75Eth
then min_effective_balance = 9.5Eth + 0.75Eth = 10.25Eth

Suppose proposer effective_balance == 10Eth,
Thus his minimal possible balance would be 9.75Eth.
Despite his balance would cover blob gas cost he still would be filtered out by the min_effective_balance =10.25Eth

[Nashatyrev]

The latter is not critical though and could be fixed later after covered with reference tests

[djrtwo]

Let's patch as is and work through the other issue separately.
Thanks!