Run embeddable migrations on read only

[rudolf]

As part of #144035 we want to split saved objects into their own indices #104081. The goal here is to prevent a migration of one type of saved object from impact all other saved objects.

Because cases embeds lens visualisations, should lens ever specify an embeddable migration, we would also have to upgrade all cases. This means we're not achieving the isolation we hoped to achieve.

Because of the sharing to multiple spaces migration that rewrote _id's we had to extract the references inside all embeddables and migrate all embeddables to ensure that rewritten _ids were updated inside the embeddables too.

Given that we're no longer going to allow new types to do a convertToMultiNamespaceType migration #147344 there should be no reason to to migrate the embedded documents. We could keep embedded documents in their old version and only migrate them on reading. This means we can't query the embedded documents for e.g. telemetry (already an anti-pattern) but the plugin that e.g. renders that embeddable will always receive the embeddable SO as the latest version.

[elasticmachine]

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

[rudolf]