[SO Migration] add an API to notify that a type is no longer used

[pgayvallet]

At the moment, if a SO type is no longer registered, it will cause the migration to the next version to fail, as we’re checking for unknown doc types during one of the migration’s preflight checks.

This already occurred once with the tsvb-validation-telemetry type in #95617

Our only workaround for now is to update an internal core variable containing an ES filter of all the documents we should ignore during the migration.

kibana/src/core/server/saved_objects/migrations/core/elastic_index.ts

Lines 35 to 43 in d920682

	export const excludeUnusedTypesQuery: estypes.QueryDslQueryContainer = {
	bool: {
	must_not: [
	// https://github.com/elastic/kibana/issues/91869
	{
	term: {
	type: 'fleet-agent-events',
	},
	},

The core savedObjects service should either extend the current registerType API, or create a new registerDeprecatedType one, to let type owners have proper control on type deprecation/removal.

To future-proof us against potential type renaming and such, I think we should also ask the owners to specify the version the type was removed at, so the API could look like

registerDeprecatedType(typeName: string, since: VersionString);

[elasticmachine]

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