Fulu BlobSidecarsByRange/Root Deprecation Support

[SunnysidedJ]

While implementing Lighthouse to avoid serving Fulu slots with BlobSidecarsByRange RPC, @dapplion and I found a need to clarify the deprecation process in the consensus specs. This PR proposes a clearer deprecation transition: allow blob sidecars for pre-Fulu slots within the availability window to be served until full deprecation (i.e., FULU_FORK_EPOCH + MIN_EPOCHS_FOR_BLOCK_REQUESTS or roughly 18 days after the fork). If the request includes Fulu or post-Fulu slots, clients may return an empty list. To ensure a smooth transition, clients should not penalize peers for requesting blob sidecars from Fulu.

The wording and structure of this deprecation policy are informed by the approach used for BeaconBlocksByRange and BeaconBlocksByRoot deprecations in phase0/p2p-interface.md, for consistency across RPCs.

Related discussions:
• Lighthouse issue: sigp/lighthouse#7122 (comment)
• PR discussion: sigp/lighthouse#7328 (comment)

[jtraglia]

A couple very minor nits. The specifications part is spot on though, thank you!

[jtraglia]

Looks great, thanks again!

[dapplion]

For reviewers, three options we can consider:

1. reject requests that include slots in range [fulu_fork_slot..]
2. reject requests that include only slots in range [fulu_fork_slot..]
3. do not reject any request (this PR)