[Discuss][Elastic Agent] Deprecation logs for Elastic Agent

Elastic Stack products will start to ship deprecation logs to a specific index based on the new indexing strategy. Elastic Agent should do the same and ship deprecation logs to `logs-deprecation.elastic.agent-*`. It should be also discussed how and where the deprecation logs of the processes are shipped.

No files, tests, or entry points are named. First clarify the indexing strategy for Elastic Agent deprecation logs and decide where deprecation logs from its processes should be shipped; the work is done when those destinations and expected behavior are agreed and documented or implemented.

Written by the indexing model from the issue text.