Migrate from legacy client to opensearch client

[zhongnansu]

Legacy ES client is still used in many places in OSD. E.g.

OpenSearch-Dashboards/src/plugins/index_pattern_management/server/routes/resolve_index.ts

Lines 59 to 64 in 730a75a

	const result = await context.core.opensearch.legacy.client.callAsCurrentUser(
	'transport.request',
	{
	method: 'GET',
	path: `/_resolve/index/${encodeURIComponent(req.params.query)}${
	queryString ? '?' + new URLSearchParams(queryString).toString() : ''

Since opensearch js client is available, do we have a plan for OSD core to migrate to use new client, and remove legacy client?
For example the above code in index_pattern_management can be migrated to something like below, that uses new client.

  const { body } = await context.core.elasticsearch.client.asCurrentUser.indices.resolveIndex({
        name: req.params.query,
        expand_wildcards: req.query.expand_wildcards || 'open',
      });
      return res.ok({ body });

[kristenTian]

[Triage]: No plan yet but we need one for migrating off the legacy one.
This will be with a scope of a project but not a quick task.

@seanneumann @ahopp

[zhongnansu]

add @dagneyb