Update package README template from Agentless to Elastic Managed integrations

[moxarth-rathod]

Summary

The integration README scaffold still documents Agentless deployment. Existing packages were updated to Elastic Managed enabled integration wording and docs links in elastic/integrations#20987. New packages created with elastic-package create package still get the old Agentless section.

This was raised on that PR: elastic/integrations#20987 (comment)

Source

internal/packages/archetype/_static/package-docs-readme.md.tmpl (currently around the Agentless commented block):

elastic-package/internal/packages/archetype/_static/package-docs-readme.md.tmpl

Lines 41 to 47 in 2a1e840

	{{/* If agentless is available for this integration, include the following section. You can determine if agentless is available for this integration by checking the `manifest.yml` file, and looking for the existence of `policy_templates.deployment_modes.agentless.enabled: "true"`.
	### Agentless deployment
	Agentless deployments are only supported in Elastic Serverless and Elastic Cloud environments. Agentless deployments provide a means to ingest data while avoiding the orchestration, management, and maintenance needs associated with standard ingest infrastructure. Using an agentless deployment makes manual agent deployment unnecessary, allowing you to focus on your data instead of the agent that collects it.
	For more information, refer to [Agentless integrations](https://www.elastic.co/guide/en/serverless/current/security-agentless-integrations.html) and [Agentless integrations FAQ](https://www.elastic.co/guide/en/serverless/current/agentless-integration-troubleshooting.html)
	*/}}

What to change

Replace the commented Agentless block with the Elastic Managed copy already used in integrations (for example packages/filelog_otel/_dev/build/docs/README.md).

cc: @mrodm