.Net: Remove the Agent preview suffix (#11303)

### Motivation and Context

Remove the Agent preview suffix

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: markwallace-microsoft

dotnet/src/Agents/Abstractions/Agents.Abstractions.csproj

@@ -6,7 +6,6 @@
     <RootNamespace>Microsoft.SemanticKernel.Agents</RootNamespace>
     <TargetFrameworks>net8.0;netstandard2.0</TargetFrameworks>
     <EnablePackageValidation>false</EnablePackageValidation>
-    <VersionSuffix>preview</VersionSuffix>
   </PropertyGroup>
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />

dotnet/src/Agents/Core/Agents.Core.csproj

@@ -7,7 +7,6 @@
     <TargetFrameworks>net8.0;netstandard2.0</TargetFrameworks>
     <NoWarn>$(NoWarn);SKEXP0110;SKEXP0001</NoWarn>
     <EnablePackageValidation>false</EnablePackageValidation>
-    <VersionSuffix>preview</VersionSuffix>
   </PropertyGroup>
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />