[FEATURE] Improve Opensearch Dashboards' Index Template Page

[CaptainDredge]

Is your feature request related to a problem?
There are two index template APIs at the moment, _templates API and _index_template API. The former is the legacy endpoint, while the latter is the new(er) composable index template endpoint. Legacy index templates (_template) were replaced by composable templates (_index_template) starting with Elasticsearch 7.8

If you create a new index template with the _template API, it will not be visible in responses of the _index_template API. Most importantly, there may be index templates currently in use, that you can't see in Opensearch Dashboards' Index Templates UI due to the page only querying the _index_templates API:

index-management-dashboards-plugin/public/pages/ComposableTemplates/utils/services.ts

Line 14 in 3c2a9a4

path: "/_index_template/*",

This has several implications:

• It is confusing for customers who have migrated from older versions and (some of) their index templates suddenly disappear from the UI.
• Makes investigations a lot more difficult for those who use Dashboards to manage Opensearch.
• Several old Elastic products that we support, such as Beats 7.12.1 [1], use the legacy _templates API to create their dynamic templates and therefore said templates are invisible to customers even though they affect data ingestion.
• It's just not right to have a UI to show Index Templates but then only show a subset of the templates.

What solution would you like?
We can have two separate sections for the two template types, legacy and current

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
Add any other context or screenshots about the feature request here.

[dblock]

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