Saved Objects should reduce the migrationVersion field count

[rudolf]

Related to #43673

The Saved Objects migrationVersion object field used for Saved Object migrations adds a dynamic mapping to detect outdated documents:

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

Lines 132 to 137 in 61a69f3

	migrationVersion: {
	// Saved Objects can't redefine dynamic, but we cheat here to support migrations
	// @ts-expect-error
	dynamic: 'true',
	type: 'object',
	},

This mapping definition can add as many as 2*Saved Object Type Count or ~114 fields. This can be reduced in two ways:

1. Change the dynamic mapping template https://www.elastic.co/guide/en/elasticsearch/reference/current/dynamic-templates.html so that dynamic string fields aren't mapped to a string/keyword multi type (2 fields) but only to a keyword field. This will reduce the mappings to 1*Saved Object Type Count
2. Remove the dynamic mapping altogether. The underlying design for Saved Object migrations allows for any saved object type to define migrations on any other saved object type. This required a map of migrationVersions

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

   Lines 170 to 186 in bf04235

   	const response = await callCluster('count', {
   	body: {
   	query: {
   	bool: {
   	should: Object.entries(migrationVersion).map(([type, latestVersion]) => ({
   	bool: {
   	must: [
   	{ exists: { field: type } },
   	{ bool: { must_not: { term: { [`migrationVersion.${type}`]: latestVersion } } } },
   	],
   	},
   	})),
   	},
   	},
   	},
   	index,
   	});

   However, this functionality has never been leveraged and the NP API's prevent defining a migration for a different type of saved object. So migrationVersion could be changed to a single field and the query could limit results to the saved object type and a migrationVersion field which doesn't match the current version (maybe we can leverage SEMVER fields if available WIP First draft for version field elasticsearch#58256)

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)