chore: rename caches and related functions to indicate finalized

[ensi321]

Motivation

EIP 6110 breaks the assumptions that validator indices are always finalized, and thus introduces pubkey caches for unfinalized validators (unfinalizedPubkey2index and unfinalizedIndex2pubkey).

As such, the original pubkey caches also need to update their naming to indicate they only store pubkeys and indices for finalized validators.

Description

Rename the following:

• epochCache.pubkey2index -> epochCache.finalizedPubkey2index
• epochCache.index2pubkey -> epochCache.finalizedIndex2pubkey

cc. @g11tech @tuyennhv

Related issue and PR: #6341 #6042