.Net: Filters graduation

[dmytrostruk]

Motivation and Context

Resolves: https://github.com/microsoft/semantic-kernel/issues/5409

1. Removed experimental flags on FunctionInvocationFilter and PromptRenderFilter.
2. Updated documentation.

Contribution Checklist

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