.Net: Update Microsoft.SemanticKernel packages to 1.71.0 (#13571)

### Motivation and Context

Bumps external `Microsoft.SemanticKernel` package references to the
latest released version 1.71.0.

### Description

- Updated `dotnet/Directory.Packages.props`:
  - `Microsoft.SemanticKernel.Abstractions`: `1.70.0` → `1.71.0`
  - `Microsoft.SemanticKernel.Connectors.OpenAI`: `1.70.0` → `1.71.0`
  - `Microsoft.SemanticKernel.Core`: `1.70.0` → `1.71.0`

> `Microsoft.SemanticKernel.Planners.OpenAI` remains pinned at
`1.47.0-preview` (separate versioning scheme). `nuget-package.props`
already had `VersionPrefix=1.71.0` (packages produced by this repo) with
`PackageValidationBaselineVersion=1.70.0` (previous stable baseline) —
no changes needed there.

### 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 😄

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Update Microsoft.SemanticKerne packages to
1.71.0</issue_title>
> <issue_description>
> Update dollowing dependencies to use version 1.71.0
> 
> <PackageVersion Include="Microsoft.SemanticKernel.Abstractions"
Version="1.70.0" />
> <PackageVersion Include="Microsoft.SemanticKernel.Connectors.OpenAI"
Version="1.70.0" />
> <PackageVersion Include="Microsoft.SemanticKernel.Core"
Version="1.70.0" /></issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #13570

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: markwallace-microsoft <127216156+markwallace-microsoft@users.noreply.github.com>

Author: Copilot

dotnet/Directory.Packages.props

@@ -80,9 +80,9 @@
     <PackageVersion Include="Microsoft.ML.OnnxRuntime" Version="1.23.2" />
     <PackageVersion Include="Microsoft.ML.OnnxRuntime.Gpu" Version="1.23.2" />
     <PackageVersion Include="Microsoft.ML.Tokenizers.Data.Cl100kBase" Version="2.0.0" />
-    <PackageVersion Include="Microsoft.SemanticKernel.Abstractions" Version="1.70.0" />
-    <PackageVersion Include="Microsoft.SemanticKernel.Connectors.OpenAI" Version="1.70.0" />
-    <PackageVersion Include="Microsoft.SemanticKernel.Core" Version="1.70.0" />
+    <PackageVersion Include="Microsoft.SemanticKernel.Abstractions" Version="1.71.0" />
+    <PackageVersion Include="Microsoft.SemanticKernel.Connectors.OpenAI" Version="1.71.0" />
+    <PackageVersion Include="Microsoft.SemanticKernel.Core" Version="1.71.0" />
     <PackageVersion Include="Microsoft.SemanticKernel.Planners.OpenAI" Version="1.47.0-preview" />
     <PackageVersion Include="Microsoft.VisualStudio.Threading" Version="17.14.15" />
     <PackageVersion Include="Microsoft.AspNetCore.SignalR.Client" Version="10.0.2" />