Post-electra cleanup

[nflaig]

Just listing out some items that can be cleaned up post-electra since we know that all clients / other software will have to be updated to be compatible with the fork (feel free to extend the list)

• remove workaround to fix go-eth2-client issue packages/beacon-node/src/api/rest/base.ts#L142-L151
• remove workaround to fix checkpointz accept header handling packages/cli/src/networks/index.ts#L177-L179
• remove workaround to get fork from state bytes instead of header packages/cli/src/networks/index.ts#L195-L198
• remove Eth1Provider, it won't work anyways once history expiry happens on EL side packages/beacon-node/src/eth1/provider/eth1Provider.ts#L54
• remove apis no longer part of spec, such as Remove deprecated produce block apis #7410
• remove if-else and only call v2 attestation apis, see example packages/validator/src/services/attestation.ts#L239-L247

[philknows]

Should probably include these also:

• Enable electra in sim tests #7604
• Update CL spec references to v1.5.0 once release is out #7463
• Review electra type safety #6799
• Update weak subjectivity period calculation for Electra #7645

[nflaig]

Should probably include these also:

those are bit different imo, it's not clean up related but rather remaining todos, but sure can list them here to have broader overview