Revert fork version fallback used by `/eth/v1/beacon/blob_sidecars/{block_id}`

[michaelsproul]

After v7.1.0 and before Fulu, we should revert the change introduced in this PR:

• Fix blob download from checkpointz servers #7495

More background:

• Add fork version fallback for GET /eth/v1/beacon/blobs #7489

This will depend on checkpointz being updated and that update being widely adopted. I don't think there is an open PR to fix checkpointz yet, but I've opened an issue:

• Serve metadata on /eth/v1/beacon/blob_sidecars/{block_id} endpoint ethpandaops/checkpointz#210

[michaelsproul]

Checkpointz is now patched as of v0.28.0:

• fix: Data versions in api responses ethpandaops/checkpointz#212

• https://github.com/ethpandaops/checkpointz/releases/tag/v0.28.0

[chong-he]

Wondering do we have to wait until all checkpointz providers to update?

I have tested removing the changes introduced in #7495, all checkpoint sync endpoints work except for: https://mainnet-checkpoint-sync.attestant.io/ which is still on the previous version of checkpointz. It errors with:
CRIT Failed to start beacon node reason: "Error fetching finalized blobs from remote: InvalidJson(Error(\"missing field version\", line: 1, column: 264155))"

[michaelsproul]

Hi @mcdee, not urgent but if you could get the checkpointz version updated on mainnet-checkpoint-sync.attestant.io it will help us revert a hack we added to maintain compatibility with old checkpointz versions that weren't following the spec (I know you like specs ❤️ ). Thank you 🙏

[michaelsproul]

Attestant still not patched so bumping this to v8.0.0