raft: Modify PartitionRestructureMigration to use PartitionKey

Context

In #6797 (closed), we're replacing the composite PartitionKey with an opaque identifier. The identifier will be a SHA256 hash of the authority storage name and partition ID.

Recently, we introduced a partition restructure migration (#6647 (closed)) which runs on startup and renames entries within the /partitions directory to be compatible with Raft. The renamed directories are essentially based off a hash of the storage name and partition ID of each migrated partition.

Proposal

We can reuse the NewPartitionKey() constructor introduced in !7974 (merged) and use the resulting hash directly as the name of the migrated directory.

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