Add fork version fallback for `GET /eth/v1/beacon/blobs`

[michaelsproul]

In our effort to follow the beacon API spec, we followed it a bit too hard and broke compatibility with checkpointz.

The issue is, since we refactored our fork-version handling, Lighthouse is now strict about receiving a version field in responses on /eth/v1/beacon/blobs/{block_id} which we use for checkpoint sync. The result is that checkpoint sync fails with checkpointz, because the server doesn't include the version field despite it being mandatory.

The fork versioning change was made in:

• ContextDeserialize and Beacon API Improvements #7372

To maintain backwards-compatibility for v7.1.0 we should consider relaxing our version parsing by using the Electra fork by default if none is specified. This is pretty janky, so we should also delete this default once enough checkpointz instances are updated.

Thanks to @barnabasbusa for reporting!

[jimmygchen]

Completed in #7495