Docs: Add additional step/check to the Loki HELM Migration Guide for checking Schema Config

[SeamusGrafana]

For the Loki HELM Migration Guide it would be beneficial to add an additional step/check into this, for the user to verify the Schema Config has been set correctly in the new HELM Chart Config, and any old Schema Configs are migrated over correctly.

This helps when users have multiple schemas that were used before of different values to those specified in the loki HELM Chart. Items to check should include, a correct from date, store, schema and index prefix etc.

Link to Doc: https://grafana.com/docs/loki/latest/installation/helm/migrate-from-distributed/

Relevant Context: https://github.com/grafana/support-escalations/issues/5824

[trevorwhitney]

I think the ideal solution here is to mention they should not change the schema when also doing a migration. Do the migration first, then upgrade the schema in the new cluster only.

Upgrading schemas and doing migrations are both complicated, do one at a time.