[2.0] Enable SlidingWindowConversationManager.per_turn by default

[zastrowm]

Problem Statement

In #1374 we added a new SlidingWindowConversationManager.per_turn parameter which added more proactive management of the context, though we did not turn it on by default for backwards compatibility; we should consider doing so in the next major release

Use Case

Reducing context for customers using agents in longer loops