lint: teach TestRaftCopyrightHeaders about new additions to pkg/raft

[arulajmani]

Previously, this would crap out because the original etcd apache header wasn't present on the file. We should be using the regular BSL header, as opposed to the etcd apache header with a CRL modified copyright, whenever we add new files to package raft. This patch teaches the linter this behaviour.

Epic: none

Release note: None