Incorrect case for `version` in new Electra `/eth/v2/validator/aggregate_attestation` endpoint response
**Describe the bug**

Hey team, looks like Nimbus returns "Electra" instead of the lowercase "electra" in the response of this new endpoint.

Nimbus response:

`GetAggregatedAttestationV2Response: {"version":"Electra","data":{"aggregation_bits":"0xfb1f","data":{"slot":"152","index":"0","beacon_block_root":"0x56288eb272e41687635cb16e2622cf801363dec0ac36635c6fdeed955bce0f89","source":{"epoch":"3","root":"0x639bb843d420b0903f9a696ac93e89bc6fdcbad81d1e2adcc0f2133f04a84834"},"target":{"epoch":"4","root":"0x94cf7908b3fe25441f92ed8fcd6b9bde4160f5ce561b5405f615deeb68714a85"}},"signature":"0xb9492a06358120a5333f0261c1c740a6a2404f34773f3bcec6fe323364cdebbd6a4286a59ace27e4d8b64c795db3563e152901eb8ca0a6cdf9be99fbd3077ecf21bbf66684aeea73a47015ad7dd0dec1d88a78178813028e9bb44af320b3d514","committee_bits":"0x0100000000000000"}}`

Teku response:

`GetAggregatedAttestationV2Response: {"version":"electra","data":{"aggregation_bits":"0xfb1f","data":{"slot":"152","index":"0","beacon_block_root":"0x56288eb272e41687635cb16e2622cf801363dec0ac36635c6fdeed955bce0f89","source":{"epoch":"3","root":"0x639bb843d420b0903f9a696ac93e89bc6fdcbad81d1e2adcc0f2133f04a84834"},"target":{"epoch":"4","root":"0x94cf7908b3fe25441f92ed8fcd6b9bde4160f5ce561b5405f615deeb68714a85"}},"signature":"0xb9492a06358120a5333f0261c1c740a6a2404f34773f3bcec6fe323364cdebbd6a4286a59ace27e4d8b64c795db3563e152901eb8ca0a6cdf9be99fbd3077ecf21bbf66684aeea73a47015ad7dd0dec1d88a78178813028e9bb44af320b3d514","committee_bits":"0x0100000000000000"}}`


**To Reproduce**
Steps to reproduce the behavior:
1. Platform details (OS, architecture): 'N/A'
2. Branch/commit used: I used the latest `ethpandaops/nimbus-eth2:unstable` image (as posted in the #interop channel on Discord)
3. Commands being executed: `nimbus_beacon_node --non-interactive=true [...]` (standard Kurtosis config for Nimbus CL)
4. Relevant log lines: 'N/A'

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**Additional context**
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