[.Net][Feature Request]: Deprecate GPTAgent

[LittleLittleCloud]

Is your feature request related to a problem? Please describe.

GPTAgent is merely OpenAIChatAgent plus OpenAIMessageConnector

Describe the solution you'd like

Deprecate GPTAgent from AutoGen.OpenAI

Additional context

Migration guide

use OpenAIChatAgent plus message connector to replace GPTAgent

// before
var agent = new GPTAgent(...)

// after
var agent = new OpenAIChatAgent(...)
    .RegisterMessageConnector()