Revert "ssz: bump (#5996)" (#6037)

This reverts commit 21601f59e2.

Reason: `/eth/v1/beacon/light_client/updates?start_period=0&count=128`
provides malformed data. Reverting fixes it locally.

Download JSON and SSZ, then use this script to validate (Holesky).
Need to export some symbols for parsing.

```nim
import
  stew/io2,
  ./beacon_chain/spec/[digest, forks, helpers, presets],
  ./beacon_chain/spec/eth2_apis/[eth2_rest_serialization, rest_light_client_calls]

let jsonBytes = io2.readAllBytes("/Users/etan/Downloads/json_updates.json").get()

let
  (cfg, unknowns) = readRuntimeConfig("/Users/etan/Documents/Repos/nimbus-eth2/vendor/holesky/custom_config_data/config.yaml")
  genesis_validators_root = Eth2Digest.fromHex("0x9143aa7c615a7f7115e2b6aac319c03529df8242ae705fba9df39b79c59fa8b1")
  forkDigests = newClone ForkDigests.init(cfg, genesis_validators_root)

let updates = seq[ForkedLightClientUpdate].decodeJsonLightClientObjects(jsonBytes, cfg, forkDigests)

var entries = newSeqOfCap[RestVersioned[ForkedLightClientUpdate]](updates.len)
for update in updates:
  let
    contextEpoch = withForkyUpdate(update):
      when lcDataFork > LightClientDataFork.None:
        forkyUpdate.contextEpoch
      else:
        continue
    contextFork = cfg.consensusForkAtEpoch(contextEpoch)
  entries.add RestVersioned[ForkedLightClientUpdate](
    data: update,
    jsonVersion: contextFork,
    sszContext: forkDigests[].atconsensusFork(contextFork))

let res =
  block:
    var default: seq[byte]
    try:
      var stream = memoryOutput()
      for e in entries:
        withForkyUpdate(e.data):
          when lcDataFork > LightClientDataFork.None:
            var cursor = stream.delayFixedSizeWrite(sizeof(uint64))
            let initPos = stream.pos
            stream.write e.sszContext.data
            var writer = SszWriter.init(stream)
            writer.writeValue forkyUpdate
            cursor.finalWrite (stream.pos - initPos).uint64.toBytesLE()
      stream.getOutput(seq[byte])
    except SerializationError:
      default
    except IOError:
      default

let sszBytes = io2.readAllBytes("/Users/etan/Downloads/ssz_updates.ssz").get()

let sszUpdates = seq[ForkedLightClientUpdate].decodeSszLightClientObjects(res, cfg, forkDigests)
doAssert updates.len == sszUpdates.len
for i in 0 ..< updates.len:
  doAssert updates[i].kind == sszUpdates[i].kind
  withForkyUpdate(updates[i]):
    when lcDataFork > LightClientDataFork.None:
      doAssert forkyUpdate == sszUpdates[i].forky(lcDataFork)

doAssert sszBytes == res
```

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