.Net: Removed experimental flags for stable items

[dmytrostruk]

Motivation and Context

Resolves: #12982

Removed experimental flags in following items:

1. ChatClientExtensions
2. ChatClientBuilderExtensions
3. AIFunctionExtensions
4. ChatCompletionServiceExtensions
5. AuthorName property in ChatMessageContent and StreamingChatMessageContent models.
6. KernelFunctionExtensions
7. KernelPlugin

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Merge failed due to unrelated integration test failure

[xUnit.net 00:06:35.62]         --- End of stack trace from previous location ---
  Failed SemanticKernel.IntegrationTests.Agents.CommonInterfaceConformance.AgentWithTextSearchBehaviorConformance.AzureAIAgentWithTextSearchProviderTests.TextSearchBehaviorStateIsUsedByAgentInternalAsync(question: "What was Acme Corp's revenue in Q1 2025?", expectedResult: "12.5", ragResults: ["Acme Corp reported a revenue of $12.5 million in Q"···, "This represents a 15% increase from Q4 2024."]) [3 m 59 s]
  Error Message:
   Azure.RequestFailedException : Service invocation timed out. 
Request: POST obotoken.vienna-eastus.svc/obotoken/v1.0/saveusertoken/v2 
 Message: Unable to read data from the transport connection: Operation canceled. Time waited: 00:00:10.0006724