Drop R&D note on span batch specs

## Overview

The span batch specification still has an "in R&D" note:

https://github.com/ethereum-optimism/specs/blob/main/specs/span-batches.md#L32-L34

This should be removed. Span batches have been live on Sepolia and Goerli for a while now, and Delta is scheduled for mainnet.

Open specs/span-batches.md around lines 32-34 and confirm the "in R&D" note is the text targeted by this issue. Remove that note and review the surrounding Markdown; done means the outdated note is gone while the rest of the span-batch specification remains intact.

Written by the indexing model from the issue text.