require that Electra produceAttestationData AttestationData have 0 committee_index (#511)

* require that Electra produceAttestationData AttestationData have 0 committee_index

* deprecate produceAttestationData

* don't deprecate produceAttestationData

---------

Co-authored-by: Nico Flaig <nflaig@protonmail.com>

Author: tersec

apis/validator/attestation_data.yaml

@@ -5,7 +5,8 @@ get:
   operationId: "produceAttestationData"
   summary: "Produce an attestation data"
   description: |
-    Requests that the beacon node produce an AttestationData.
+    Requests that the beacon node produce an AttestationData. For `slot`s in
+    Electra and later, this AttestationData must have a `committee_index` of 0.
 
     A 503 error must be returned if the block identified by the response
     `beacon_block_root` is optimistic (i.e. the attestation attests to a block