Add update hook to backfill the source_ids_hash index on migrate_message_* tables created before #3085192

Problem/Motivation

#3085192 added 'indexes' => ['source_ids_hash' => ['source_ids_hash']] to the message table schema, but only for newly created tables. Sites whose message tables predate that commit never receive the index. Sql::delete() filters on source_ids_hash, so every delete full-scans the message table.

Steps to reproduce

Proposed resolution

mirror the existing migrate_update_10100() pattern:

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet