.Net: Update Microsoft.SemanticKernel packages from 1.68.0 to 1.70.0

[Copilot]

Motivation and Context

Updates Semantic Kernel package references to align with the latest 1.70.0 release.

Description

Updated three package versions in dotnet/Directory.Packages.props:

• Microsoft.SemanticKernel.Abstractions: 1.68.0 → 1.70.0
• Microsoft.SemanticKernel.Connectors.OpenAI: 1.68.0 → 1.70.0
• Microsoft.SemanticKernel.Core: 1.68.0 → 1.70.0

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 😄

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update the Microsoft.SemanticKernel packages from 1.68.0 to 1.70.0</issue_title>
<issue_description>In this file https://github.com/microsoft/semantic-kernel/blob/main/dotnet/Directory.Packages.props

Update the Microsoft.SemanticKernel packages from 1.68.0 to 1.70.0
</issue_description>

<agent_instructions>Update the Microsoft.SemanticKernel packages from 1.68.0 to 1.70.0</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .Net: Update the Microsoft.SemanticKernel packages from 1.68.0 to 1.70.0 #13523

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