Migrations v2: Ensure index is lowercase

[tylersmalley]

The Elasticsearch snapshots contain a SNAPSHOT version qualifier and needs to be lowercased.

  log   [20:21:58.029] [error][savedobjects-service] [.kibana] [invalid_index_name_exception]: Invalid index name [.kibana_8.0.0-SNAPSHOT_001], must be lowercase
  log   [20:21:58.030] [error][savedobjects-service] [.kibana] migration failed, dumping execution log:
  log   [20:21:58.031] [info][savedobjects-service] [.kibana] INIT RESPONSE
  log   [20:21:58.033] [info][savedobjects-service] [.kibana] INIT -> CREATE_NEW_TARGET
  log   [20:21:58.040] [fatal][root] Error: Unable to complete saved object migrations for the [.kibana] index. Please check the health of your Elasticsearch cluster and try again. ResponseError: invalid_index_name_exception
    at migrationStateActionMachine (/usr/share/kibana/src/core/server/saved_objects/migrationsv2/migrations_state_action_machine.js:141:11)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async Promise.all (index 0)
    at SavedObjectsService.start (/usr/share/kibana/src/core/server/saved_objects/saved_objects_service.js:163:7)
    at Server.start (/usr/share/kibana/src/core/server/server.js:283:31)
    at Root.start (/usr/share/kibana/src/core/server/root/index.js:58:14)
    at bootstrap (/usr/share/kibana/src/core/server/bootstrap.js:100:5)
    at Command.<anonymous> (/usr/share/kibana/src/cli/serve/serve.js:169:5)
  log   [20:21:58.045] [info][plugins-system] Stopping all plugins.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[liza-mae]

This has broken master snapshot across our testing.

[pgayvallet]

cc @rudolf